Thursday, December 01, 2016

Embedded challenges

Written by  Anthony Fazio, P.E., and Jonathan Schneider, P.E., SEPTA
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It was once a common practice for freight or passenger rail service to traverse paved or cobblestone streets. Yet today, there are still many places in North America that retain such type of infrastructure for regular railroad or light rail service.

For example, consider Amtrak’s California Zephyr, which still operates down a bustling street in Oakland, Calif., or the numerous freight spurs in the streets of small towns. Most important to point out is the emergence of a new light rail era that almost always requires some street-running track to ensure healthy ridership.

When considering design and construction of street-running track, there are some basic fundamentals to consider that do not normally apply to ballasted track in exclusive right‐of-way.

fazio CompositeAs the old saying goes, “the three most important things to know about track are drainage, drainage and drainage.” This adage holds true for street-running track, maybe even more so than ballasted track. The key here is the obvious: A level roadway will allow standing water that freezes, which causes damage and creates safety hazards. The track design (or redesign for existing track) must be consistent with roadway drainage and road drains.

However, following the “crown” in the road can lead to cross-level issues on tangent track. Some designers have gotten creative and put a slight roadway crown inside the track gauge only. Other designers use zero crown and have installed elaborate trench drain systems, such as Hudson Bergen Light Rail (HBLR) in Jersey City. In the case of HBLR, the drainage gradient is in the direction of LRT traffic.

Another technique is to level both the track and roadway and grade the roadway slightly away from the track’s field rail only. It’s important to point out that conforming to a roadway crown within double-track street-running territory would introduce a cross-level into the track structure that may or may not be acceptable for the rail service’s timetable speed, and will accelerate rail head wear. Also, note that roadways within a single municipality can fall under the jurisdiction of a state, county, or the municipality itself, so a working relationship with the respective authorities is imperative.

Standard running rail in the U.S. is tee rail, even for in-street track, generally speaking. At one time, girder rail was very common for street track and was readily available in the U.S.; this is no longer the case. Girder rail is essentially a running rail with a guard rolled into it. There were many different sections manufactured, each with a different combination of railhead profile, flangeway depth, web, etc. Essentially, the girder allowed for a continuous flangeway.

A common practice in contemporary new construction is to use standard tee rail and form a flangeway into the concrete pour. Other common practices are to use precast concrete panels with a flangway formed into the precast panel and even timber “log guards” to form a flangeway. Both types of rail design have advantages and disadvantages. The most noteworthy comparison is that girder rail is difficult to procure and difficult to pre‐bend, yet easer to install and offers redundancy of a fully guarded running rail system. Note that it is essential to select a rail whose head is integrated with the profile of the train’s wheel (or vice versa).

Other track devices unique to street bound railways are “tongue and mate” switches and flange-bearing frogs. These devices reduce roadway deviations for automobile traffic and pedestrians. They also offer smooth wheel to rail interface. Note that traditional streetcar tongue switches have a shorter “throw” than current railroad tongue switches.

Not only must trains, cars and pedestrians share the road, but so must public utilities. Utilities present their own unique challenges. Sewer, water, telephone, gas and commercial electric lines must be considered in initial design, and reconstruction, of street running track. Also important is their accessibility. For example, SEPTA, in many locations, has manholes located generally in the gauge of its street-running track. At times, slight horizontal curves are introduced for the sole purpose to avoid costly relocation of utilities, some of which have been in place for more than 100 years. The tradeoff in this example is higher railway maintenance costs in the future with a less-than-perfect alignment for lower construction costs.

Traditional railroaders prefer ballasted deck bridges. The ballasted deck allows for seamless interface between the track structure and the bridge structure and the respective departments that install and maintain them. Other methods used to integrate the track and bridge structures include low-restraint rail clips on open-deck bridges that allow the bridge to move under the rail, and expansion rails, dependent on design characteristics.

Street-running embedded track, however, can enjoy neither of these methodologies to relieve stress. A system used when an under-grade bridge must support both the rail and the automobile traffic are expansion miter rails. They allow for proper interaction of rail, roadway and bridge. The concept here is that they are placed in the rail system above the bridge’s expansion bearing. The bridge is free to move, and the rail moves with the bridge deck and superstructure, as opposed to the rail fighting the bridge (the rail is locked into embedded concrete on both the bridge deck and on the bridge approach).

These devices are relatively simple and maintenance‐free, as opposed to a moveable bridge’s miter rails, and insure longevity of the infrastructure by relieving residual stresses.

Track buckling countermeasures are a serious topic for railroads and light rail systems with continuous weld rail in ballasted track. Embedded track has low buckling risk. However, street‐running embedded track can still buckle. It is most prone to buckling in areas where wood tie construction is paved over with asphalt. This buckle is not in a typical “S”. Rather, it can be identified as a heave in the rail. Rail temperature should be considered when building this type of track, and track inspectors should be made aware of the signs of stressed rail in embedded track.

Street-running embedded track is once again a common subject for railway designers, constructors and maintainers. There are many different topics pertaining to street running track that can easily be overlooked. Designers and constructers must consider integration with other modes of transportation and other railway disciplines. So too must they consider maintenance, cost effectiveness, and most important, safety when implementing track in street-running territory.