Tuesday, December 09, 2014

Movable bridge best-practices

Written by  Anthony Fazio, P.E. and Lester DeLago, for Railway Age
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Even with the use of advanced technology, movable bridge machinery on an electrified railway remains a complex combination of structural, mechanical, and electrical systems.

The geography of the Northern New Jersey/New York metropolitan area, combined with the crisscrossing of this region by a plethora of rail lines, mandated many rail crossings over navigable waterways. A large number of these consist of movable bridges of various configurations—bascule, lift, and swing—the majority of which carry electrified passenger railways. Extraordinary measures are taken by a railroad’s professional and maintenance engineering staff to ensure safety.

100 0924Even with the use of advanced computer technology and electronics for controls, movable bridge machinery on an electrified railway is a combination of structural, mechanical, and electrical systems and sub-systems. While redundant system checks are used for safety (to guard against the possibility of a false clear), multiple levels of redundancy applied to a vital (fail-safe) system will reduce system availability.

While designs of individual bridges may vary, the critical mechanical and structural elements are common:

Centering devices: Primarily on swing bridges, the centering device is a large metal bar that aligns and locks the swing span to the adjacent bridge spans in a vertical direction.

Wedges: Wedge-shaped retractable bridge bearings that physically lift the swing span and transmit the load directly from the movable span to the bridge pier when the bridge is in the closed position. The wedges sit on a steel plate called wedge shoe or wedge bed. When the wedges are withdrawn, the dead load of the movable swing span completely shifts to the center bearing in order to swing the span open.

Span Locks: Large metal bars affixed from the movable bridge span and extend horizontally into a socket on the approach spans when the bridge is closed for train traffic. The span physically locks the movable span down to the adjacent fixed spans. They are particular to lift and bascule spans.

dock bridge 058Miter Rails: Rails that overlap from the movable span to the fixed span. They provide a smooth transition for the wheel of the railcar from the rail on the approach (fixed) span to the movable span. There are many different types of miter rails. Some types include High Speed Long Point Miter Rails, Standard PRR Miter Rails, and Rider Rails.

Expansion Rails: Rails used at approaches to movable bridges. Rail length has a tendency to increase with increasing temperature; the expansion rail assembly ensures that temperature changes do not change the length of the rail near the movable-to-fixed span transition area. The device allows longitudinal movement of the expanding rail to slide past a fixed and firmly anchored rail. Increased rail length and compression forces near a movable-to-fixed span transition zone, if not addressed, could disrupt miter rail-to-bridge interface tolerances, and ultimately make closing a movable span impossible.

Darlington Couplers: Large knife switches designed to carry track circuits. The blade travels with the movable span and the receiver stays with the approach span.

Catenary Sled: A large frame that extends from the movable span to overlap on the approach fixed span that supports the catenary wire. This frame retracts every time the movable span is opened. The purpose is to have an overlap in the catenary for a smooth transition of the train’s pantograph resulting in decreased electric arcing.

Interlocking: A series of devices that are interlocked for train control. The entrance to an interlocking is controlled by a signal. Interlockings have a predefined operating sequence through the use of solid-state microprocessors or complex relay circuits and electromechanical devices. Movable bridges are considered interlockings.

dock bridge 045Relay: An electrically operated switch that uses an electromagnet to move a set of contacts (or multiple sets of contacts) in order to energize another electrical circuit (or circuits). Relays are either on (energized) or off (de-energized). Relays can also be solid-state and are used extensively in engineered configurations for interlocking controls.

Track Circuit: A low-voltage electrical circuit used to check occupancy of a section of track by using the rail as its conductor and a train to ground circuit. The grounded circuit shows occupancy of that circuit by de-energizing a relay.

Operation of these devices, to the allowable error tolerances and in the prescribed sequence, must be verified prior to operating a train over the structure.


There are two groups of safety critical electrical circuits found on movable bridges for both indication and control of the bridge apparatuses: Bridge Department circuits and Signal Department circuits. The latter are part of the train control system.

Bridge Department indication circuits are essential to the primary (motor power) bridge operating circuits, whose main function is to open and close the bridge. Signal Department circuits are essential in determining if the movable bridge is ready and safe for train movement. The Bridge and Signal System indication systems are separate and operate independently of each other. They are, however, integrated in such a fashion that they inhibit the operation of certain elements of the other system, depending on the mode (opened/closed/in transition) of the bridge. For example, when a “proceed” train signal is displayed for train movement, the Signal System inhibits the bridge control circuits from operating any movable components such as miter rails and wedges. At the same time, when Bridge Systems are not in a safe position for train movement, the Signal System is inhibited from displaying favorable signals over the bridge.

spuyten duyvil wedge no1Position-indicating devices that railroads commonly use are electromechanical devices called Circuit Controllers. They indicate position of movable bridge components to a microprocessor or a relay-based control circuit. Highly robust devices, they’re well suited to the challenging environment (temperature, ice, impact load) associated with railroad movable bridges. They convert rectilinear motion, such as the movement of a shaft along its line of travel, to rotational motion, which is used to open and close electrical contacts. Indications achieved through use of Circuit Controllers are either on or off; that is, the device is either in a position that allows a circuit to be “made” or it is not. The intermediate position of the device is not measured as it might be, for example, through the use of an LVDT (Linear Voltage Displacement Transducer) and is not relevant for the function of the bridge. The logic controller only needs to know if the device has achieved the required position and clearance for the next sequence of a bridge opening operation or that the device is down and locked into position for train traffic. The sole exception to this, however, is the actual position of the movable span. This position (as rotation or translation) is always transmitted to the Bridge Tender either mechanically or electrically. For example, lift bridges are usually lifted only to what is required to allow river traffic safely under the movable span. Moving a lift span to the top of its tower will increase maintenance costs and add to opening time, causing delays in high density rail systems. Therefore, intermediate height of a lift span is pertinent information.

Bridge circuit controllers are typically utilized in low voltage circuits to indicate to the bridge logic controller the position of a movable component. This information is utilized by the logic controller to determine when higher voltage and current circuits should be either activated or deactivated, via electrical contactor, to motors that move bridge components like miter rails and wedges. In the railroad industry, Circuit Controllers that are commonly used are U5 Boxes, GRS Boxes, and Gemco Boxes.

The sequence of operation for a movable bridge, such as a swing bridge, is a series of precise and calculated events. All movable bridges on main line railroads operate under similar premises. However, a modern swing bridge is the most complex. From the moment a mariner calls the Bridge Tender for an opening, there are numerous checks that the Signal System executes electrically before the Bridge Tender’s control panel is even energized. From the perspective of railroad operating rules, the movable bridge is classified as an Interlocking; the entrance to it is governed by a controllable signal. The Bridge Tender notifies the Train Dispatcher of a request to open the bridge, who puts in a request to release the bridge for local control to the Bridge Tender.


100 0936However, the bridge cannot be released until certain criteria are established by the Signal System. First, all signals at the entrance to the bridge are at “stop.” Second, no train can be detected on the bridge or its approach. The controllable signals must be at stop for a pre-determined amount of time, so that a train approaching the bridge has adequate time to safely stop prior to reaching the stop signal. Also if a train is on the bridge, the train has the time to clear this section of railroad. The occupancy of the bridge is also checked for traffic by the track circuit relay to ensure the bridge is not released with a train sitting or traveling on it. Other features used on railroad movable bridges are derails on the approaches. They are designed to derail a train that violates a stop signal.

Once the conditions have been met by the control logic in the Signal System, the dispatcher can now “unlock” the bridge. This request is via a control circuit to check all of the required conditions. If met, the unlock relay in the bridge energizes the bridge control panel with electrical power, usually 60 Hz AC commercial power. The Signal Department power for indication of devices is typically low-voltage DC.

Also, audible alarms on movable bridges are mandatory. The alarm is considered a safety-critical item and is often directly wired to the unlock relay so that employees are alerted of impending bridge movement at the moment the bridge is unlocked to open. Less-sophisticated systems employ Standard Operating Procedures (SOPs) that require Bridge Tenders to sound alarms before any bridge opening. The purpose of the SOP is to establish a rigid and mandatory function for a Bridge Tender to replace that particular function of a pre-engineered controlled system.

At this point, the Bridge Tender has full control of the system, and the operation transitions that of the Bridge Department’s. The modern swing bridge’s control panel typically indicates whether a device is extended or retracted. All of these different systems are usually driven by electric motors and indicated for position and current by Circuit Controllers. All electric motors have a motor brake to physically lock the device they are controlling when the motor is not energized. Motor brakes are important, because devices such as wedges would push themselves out from the weight of the bridge without a brake. It is not uncommon, and is actually a benefit, to have the ability to manually crank each one of these mechanical bridge components in the event of a motor or power failure.

During the opening sequence, the first bridge subsystem energized on electrified railroads is the catenary sled subsystem. It is electrically impossible to skip one of these subsystem steps; each must be performed in a sequence in order to go to the next step to prevent physical damage to equipment (swinging a bridge with devices still engaged). Once the Bridge Tender receives indication that both catenary sleds are lifted or retracted, the bridge’s centering devices are pulled. The next step typically is to pull center wedges. However, this is predicated by structural design of the bridge. Also, some swing bridges do not have center wedges because their center bearings are designed to transmit live load. Once center wedges are pulled, the Bridge Tender pulls the end wedges and lifts the miter rails as one subsystem. Miter rails and end wedges can be operated by the same gear train and shafting, The miters are geometrically arranged to lift slightly before the wedges pull, so that they are not damaged when the bridge settles onto its center “swing bearing.” Miter rails and wedges still get their own Circuit Controllers despite sharing drive machinery.

After all these conditions have been met, the swing subsystem is nearly prepared to engage. DC drive motors, in past practice, have been favored for this subsystem function because traditional DC motors are controllable at various speeds for a smooth opening/closing. Motor-Generator Sets (MG Sets), Thyristor Drives, and Vector Drives are common motor control systems used for this phase of an opening.

The bridge has one final sequence before swinging: The “machine” brakes must be released. Machine brakes are drum or disk brakes on the main machinery shaft. Once electrically released, indication is sent to the bridge control system and the bridge can swing.

For lift and bascule type bridges, the machine brakes are not released until the energy is sent to the main drive motor through a relay that picks up within the motor circuit. The relay releases the machinery brakes while the drive motors are energized. As soon as the drive motors are de-energized for any reason, the brakes are reset. This is so that the movable span is under full control of either the motor or the brake, to control the span’s acceleration due to gravity.

A rotary cam box, typically a Gemco Box, indicates the position of the bridge with a unique gear ratio to each swing bridge for purposes of opening and closing contacts. This rotary cam box indicates when the bridge is nearly open, fully open, nearly closed, and fully closed.

At nearly open/closed, the speed of the bridge is slowed or “ramped” by the drive motor to slow the inertia of the bridge. At fully opened/closed, the power to the motor is cut off and the machine brakes are set (the brakes are momentarily released while centering device is driven after swinging closed).


Once the bridge is closed, its control is returned to the Train Dispatcher, and power from the bridge’s control system is removed via release relay. Before the Train Dispatcher can display a signal for train movement on the bridge, the Signal Department’s vital circuitry performs checks on every safety-critical device to insure that all movable components are in their proper position to allow for safe train movements. Often, the Signal Department does this by a separate set of Circuit Controllers. Only after this is confirmed will the signal system permit the movement of trains.

A noteworthy part of this process is the Signal Department’s Cycle Check on pertinent equipment such as miter rails. This insures that the equipment being checked, such as miter rails, are in fact lifted for the bridge opening and reseated after the opening, completing the cyclical process of lifting and seating. This ensures that a Signal Department Circuit Controller is not malfunctioning or seized. It is also a check on the condition of the miter rail or other Bridge Department device. Without the Cycle Check, Circuit Controllers that are seized in place could still display a signal to permit train movement, regardless of position and condition of the miter rail. The Signal Cycle Check is independent of the Bridge Department’s opening sequence.

Integration of these subsystems and devices is complex. The Signal Department requires a higher level of accuracy in its system checks than the Bridge Department does for each moving bridge apparatus to accommodate safe train movement. For example, Bridge Wedges must be driven within one inch of their seated location since they are movable devices that, when properly driven, transfer the live load of a locomotive from the bridge superstructure to the pier. Improper driving and seating could lead to derailment. This is an FRA-mandated requirement for the Signal Department. Also, miter rails that are more than 3/8-inch from the proper seated position required for train movement must not display a signal for train movement. The 3/8-inch tolerance for obstructed rail is also integrated and consistent with the Track Department’s requirements for acceptable railhead wear on miter rails. These tolerances must be tested every month by the Signal Department. The Bridge Department provides a support role in these tests.

A position-indicating device currently being used to eliminate the moving parts and contacts of Circuit Controllers are Proximity Sensors. These were originally intended to augment the conventional Circuit Controller in places like the heel of a miter rail, where high cyclical stresses are induced from the machinery, bending the rail upwards for clearance to swing. However, at some locations, they have been used as a full replacement of Circuit Controllers. These devices essentially are indicating the presence of another piece of metal in the required proximity to the Proximity Sensor by means of electrical induction.

Many railroads have worked to integrate different types of engineering in order to provide reliable and advanced designs. For example, PRR-type movable bridges utilized a machine to drive a one-inch-diameter blunt end locking rod from the movable span miter rail to the fixed side miter rail to physically lock the mating miter rails together. If the lock rod didn’t drive as a result of any misalignment, the track circuit didn’t carry to its receiver contacts housed in the Bayonet Box. A rod that did not drive after closing a bridge would not allow the Bayonet to carry the Signal Department’s track circuit and would display a stop signal. Once the rod was driven, it provided shear strength to lock the two components of the miter rails together. This particular system was ultimately phased out and replaced with systems requiring less maintenance, such as Proximity Sensors to check the position of the rail.

Proximity Sensors are an effective tool and could continue to be used as supplement to other indicating devises. Locking rods with Bayonet Boxes have been viewed as troublesome. However, at places where miter rails are restricting train speeds, miter rail lock rods should be considered as a tool to effectively remove those restrictions. Using old ideas with new innovation, such as modern drives, could create a better product.

There are other possibilities for improvements in control and indication for movable bridges. Currently, the wedge indicates from the movable span and sends the wedge position to the relay logic on the movable span. Logic indicates if the wedge is in the extended or retracted position as a surrogate measure for whether or not the wedge is seated and taking dead load, and suitable for transmitting live load. Again, wedges retracted more than one inch should only display a stop signal to trains entering the bridge limits. The current industry standard does not indicate if the bridge’s wedges are properly sitting in their shoes on the pier.

Therefore, if the wedge shoe were entirely removed, the signal system could still display a favorable signal and allow a train over a bad, missing, or misaligned shoe. This is because the wedge is extended, but not necessarily seated. An enhancement could be to install proximity sensors mounted to the shoe, so that the wedge indicates in proximity to the wedge shoe. In other words, the Proximity Sensor would also indicate the condition or existence of the shoe/pier.

Other useful devices that could be applied are Remote Thermal Sensors to check the temperature of the bridge. These devices have been used to monitor neutral rail temperature remotely in the summer months and assist in placing applicable speed restrictions. For the Bridge Department, they would be applicable for opening restrictions due to extreme temperature. Also, the use of Impact Sensors could be used on movable bridges to provide maintenance information. Although rail impact loads at miter rails have been studied on a case-by-case basis, it is not common practice to continuously monitor them. High-speed, long-point miter rails require more maintenance; constantly changing profiles in rail head from wear increase impacts on masonry piers and wreak havoc on mortar for stone masonry units, bridge timbers, steel ties, and wedges. Most common from impact loading at miter rails is damage to Circuit Controllers. Remote monitoring devices could be installed so that these impacts can be monitored remotely and trended to assist in scheduling maintenance.

Anthony FazioEvent recorders are used on some movable bridges. These are used to replay events and troubleshoot problems without having to use precious time to re-enact the failure mode. Traditional event recorders take a reasonable level of competence to use. Modern recorders can be made with easier displays and designed to be more user friendly to the Bridge Tender, not just the technician (although they are ideally one in the same job).

The writers gratefully acknowledge the technical review and comments of Steve Fiel, Irfon Oncu, P.E., and A.E. Fazio, P.E.

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