Decoding Transit Signal Labels

 As I have been covering for some time, North American railroad interlocking stations are generally well labeled. However rail transit is quite a different beast and although some systems have adopted railroad practices, the majority have settles on a pair of labeling schemes that at first may look impenetrable, but are actually pretty straightforward. To avoid burying the lead these schemes are:

• Sequential - Interlockings are numbered, often by line.
• Prefix - The interlockings have names, but a prefix is used for observable signage.
• Milepost - The interlockings may have names, but all signals show a generic milepost.

Of the three systems, sequential and prefix are easiest to decode as it is clear what is the interlocking name and what is a signal designation. Milepost is often unhelpful at identifying the real signal number and the real interlocking designation. Even within these broad bins there is aa bit of variation so here are some examples to help with the lesson.

The WMATA DC Metro is one of the more involved sequential systems. Each line segment is given a letter and interlockings are then numbered sequentially from one end of the line to another with any infill interlockings going on top of that.  Above we see the 08 signal of interlocking 03 on the "G" line at Morgan Blvd. 

The MBTA also uses a sequential system for signal markings and formal documents, although human relatable names also seem to exist. Here we see the 1N signal at interlocking #7, aka Park. Both the DC Metro and MBTA have to ensure that a signal number is not mistaken for an interlocking number so in the case of DC the interlocking has a letter prefix while the MBTA uses a directional letter suffix after the signal number.

MATRA also uses a system with somewhat sequential numbers in addition to a line prefix. These are combined with a 3 digit plus alpha signal ID that consists of a track number followed by some sort of two digit configuration or frog type (seems to be 20, 30 or 40) followed by a signal specific letter.  The signal above would be Line "S" (South), interlocking #225, track #2, type 40, signal G.

The second popular system is to use a letter prefix in front of a signal number. The prefix is usually short for a more human relatable name and can usually be decoded by lay observers.  For example the Pittsburgh Light Rail interlocking on the old PRR Panhandle Bridge station is prefixed by FS.

St. Louis Metrolink uses literally signal number prefixes on the same typographic line. This can cause some confusion for visiting observers when prefix like "S" matches a common direction prefix.

Milepost labeling is the least helpful at providing observers with hints as to the proper way to identify interlockings or other railway locations. In this example from the Denver RTD light rail we can see that interlocking signals have the exact same number plate as automatics. This consists of an S prefix for "Signal" a milepost and a line suffix like PR, CC or SW. The lack of information is somewhat mitigated by the provision of switch identification boards which provide both "lever" number and the switch specific letter suffix for paired turnouts.

The San Jose VTA light rail uses a line prefix letter followed by a milepost-y number and then a modifier like A/B or R (for reverse direction).

There are of course many light rail and transit systems all with their own variations or exceptions, like San Diego still using the L/R system found on US&S CTC consoles, but these examples should get the novice most of the way to decoding transit signal labeling.