Typical Selection Report (TSR)

Used on underpass projects, the TSR identifies materials and construction methods to be used on the railroad bridge proposed to be built or modified by TxDOT or local governments. In the rare case that an underpass bridge is being developed on a new road alignment or a road alignment with an existing at-grade crossing, the method will typically involve an at-grade shoofly track alignment that routes the railroad around the footprint of the proposed structure to allow for its construction. If this method is performed in a cut excavation, longitudinal shoring will likely be required under the influence of railroad live load.
More commonly, a railroad underpass replaces an existing underpass structure due to deficient vertical or horizontal clearance in conjunction with a safety or added capacity project. In this case, three options
are available,
 considering maintenance of rail traffic.
  • Option 1:
     Build the proposed underpass adjacent to the existing underpass and develop a revised railroad alignment that ties into the approaching railroad alignment. The railroad company may be open to this option if there is already a horizontal curve at this location or if the design speeds are low enough that a jog in the railroad alignment is acceptable. For tangent existing track alignments, the railroad company typically does not approve of providing such a permanent realignment.
  • Option 2:
     Build the proposed underpass on the same alignment as the existing and construct a bypass shoofly alignment with a temporary bridge structure (if maintenance of highway traffic is required). This temporary bridge structure can be of lower cost open deck construction with shorter spans and vertical clearance less than the final ultimate condition for the short duration of its usage. Phasing of the lower roadway construction should be considered in developing the temporary shoofly bridge and new mainline bridge layouts. A subset of the second option involves building a permanent bypass shoofly alignment. This is only entertained if it proves more cost effective overall than a temporary structure and if the railroad company cost participates in providing what effectively is provision for a future second track.
  • Option 3:
     Construct a “roll-in” where the replacement structure is built near or adjacent to the existing bridge and physically lifted or slid into place on new substructures in a short duration process that
    reduces the time
     of complete closure of the railroad (usually 72 hours maximum). While the “roll-in” technique can
    create
     significant cost savings in avoiding a temporary structure and approach track alignment, it also carries measurable construction risk considering the delay potential to the railroad company. TxDOT has had limited success in achieving railroad company approval of “roll-in” replacements, and they should only be pursued if early coordination with the railroad company is undertaken and if the railroad line has limited traffic.
Structure selection should consider the railroad company’s preferred structure selection if feasible. Refer to railroad company published guidelines for preferred structure types. Generally, railroad companies prefer multi-girder steel deck girder bridges due to weight savings (ease of removal replacement) and ease of repair. TxDOT preference is multi-girder composite prestressed girder bridges due to construction cost savings, but these have limited span capability and higher vertical clearance requirements in railroad company guidelines.
In the case of replacements involving limited vertical clearance or long span applications, a through plate girder bridge may be needed to minimize structure depth below the rail or span roadways with a large number of lanes and/or clear zone. The designer should be aware this is the railroad company’s least favorable structure type due to its fracture critical nature and expense. Clear safety improvements or cost savings (e.g., avoiding a pump station or major railroad grade raise) will need to be demonstrated to successfully gain railroad company approval of a through girder structure. Ballasted deck structures are required for all underpass projects over a roadway, and TxDOT preference is for a composite concrete deck in lieu of a steel plate deck if railroad company approval can be obtained and target vertical clearance is achievable.