The signals you see with two lights are called Home signals. Home signals are used at interlockings, that is, places where tracks connect to each other, to prevent trains from entering conflicting routes, or from going the "wrong way" on a switch set against them. Unlike other kinds of signals, home signals are absolute, that is, when red, they do not merely indicate "stop", but "stop and stay": the train operator may not pass them in this state, and the train stop (the automatic tripping arm at track level) will enforce this. Except on unresignalled portions of the original IRT (click for more info), home signals always have two heads (vertical group of two or three lights in one case) and a single yellow call-on light. Home signals are always controlled by a signal tower (a place where a human tower operator participates in signal and switch control, usually nearby), in addition to by train motion. The lower head bears a number (e.g., X 134) identifying the particular lever (whether it is a real lever or merely a simulated one) at the controlling tower that controls this particular signal. See Interlocking.

The system of indications (what the signal means) described by all of the aspects (combinations of colors of lights) below is quite simple. The upper head tells how (fast, slow, not) to go, and the lower head says which way (normal route (approximately "the straight track"), diverging route (approximately, "the switching track"), or not at all) to go. As with the other kinds of signals, a green aspect on top means to proceed at normal speed, and guarantees that the next signal is clear (not red) as well. Yellow on top means to prepare to stop at the next signal, whether it is clear or not. Red on top only appears with red on bottom, and means stop and stay. Other auxiliary aspects ("S" or "D" or lunar white of GT signals, "20" of ST signals etc.) appear below the upper head and have the same meaning as on automatic or approach signals (single-headed signals). Green on bottom means take the normal route, yellow the diverging route, and red stop. Although every "choice of routes" (facing point) switch has a home signal preceding it to indicate route, many home signals, for example, those behind the switch in the other direction, do not govern a choice of route, and may not even have a lower yellow aspect. Those home signals governing a switch where the only permitted move is over the switch will lack a lower green aspect.

Sometimes, you may see home signals not in obvious proximity to switches. These may be holdout signals, which enforce the direction of traffic on bidirectional track. Follow the link for a moderately technical description.

The special aspect Red over Red over Yellow indicates the "call-on", which is a three-step coordinated procedure between train operator and tower operator (see Train stops) permitting moves into occupied track and certain other normally disallowed moves. The 3-yellow (or "yard aspect") Yellow over Yellow over Yellow is unrelated to Yellow over Yellow, and governs movements onto yard leads and less-frequently-used tracks. The BMT/IND system of aspects and indications is used on some other rapid transit systems, such as Toronto's.

Aspects

Home Signal aspects and indications, BMT, IND, and newer IRT

Proceed on main route	Proceed on main route prepared to stop at next signal	Approach at posted speed then continue on main route (next signal is a "Two shot" GT signal, and will clear if you approach this signal at posted speed or below (whereupon this one will turn green)). See Time Signals.		Stop and stay
Proceed on diverging route	Proceed on diverging route prepared to stop at next signal	Approach at posted speed then proceed on diverging route ("Two shot" GT aspect similar to above). See Time Signals.	(3-yellow or yard aspect) Proceed with caution prepared to stop within vision expecting to find track occupied	(Call-on) Stop, operate automatic stop manual release, then proceed with caution prepared to stop within vision, expecting to find track occupied, a broken rail, or other obstruction in the block. See Train stops.

The "Old IRT" System (Unresignalled Original IRT)

The original (1904) IRT had a different system of aspects for the indications shown above, which is more like a system used on many mainline railroads. As of this writing (September, 1997), almost all of this signalling has been replaced with newer signalling using the same system of aspects as the BMT and IND, and you must go to some lengths (e.g., the White Plains Road line between its portal and E. 180 St.) to find "old IRT" signalling. There is some on the Pelham line, but the replacement signals are being installed as I write.

The "Old IRT" system is even simpler than the BMT/IND system. Each head speaks of one route, the upper head the "normal" route, the lower head the "diverging" route, the third head, if any, the yard route, if any. Thus, only one head will have an aspect other than red at once, and that says which way you go. Two reds means the same as three, or even one for that matter - an "Old IRT" home signal is recognized by its red number plate with white lettering (always bearing the lever number, 4L/X means lever 4 to the left), not by its multiplicity of heads. The number plates for different heads (and thus, different routes) of the same home signal will have number plates suggesting that they are in fact different signals, i.e., 6La/X and 6Lb/X. In the "Old IRT" system, approach signals and single-headed home signals are visually and functionally identical (although the need for each is different) -- both have red number-plates, are interlocking-controlled, and are absolute.

Call-ons are the same as with the BMT/IND system; the call-on light is low enough as to not be easily mistaken for another head. IND- and BMT-style time signal aspects have in years past been eerily planted between heads of these signals as well (ex., South of 149 St. on the Jerome line). The photograph above is of a three-headed home signal North of E. 180 St. on the White Plains Road line (September, 1997).

Home Signal aspects and indication, older IRT Division only

Proceed on main route	Proceed on main route prepared to stop at next signal	Stop and stay	(Call-on) Stop, operate automatic stop manual release, then proceed with caution, prepared to stop within vision, expecting to find track occupied, a broken rail, or other obstruction in the block. See Train stops.
Proceed on diverging route	Proceed on diverging route prepared to stop at next signal	Approach at posted speed then continue on main route ("Two shot" GT aspect -- see Time Signals.)	Proceed on 2nd diverging route prepared to stop at next signal
Proceed on main route	Proceed on main route prepared to stop at next signal	Proceed on diverging route	Proceed on diverging route prepared to stop at next signal

Dwarf Signals

Dwarf signals are used in yards, where full safety is not necessary and the expense of hundreds of signals can add up, and (decreasingly) for rarely-used reverse-direction moves (backwards entrance to interlockings) on mainline running (revenue) trackage. As the name implies, they are short in stature, and never have a green aspect. Their "most permissive" indication is always "go very slowly, prepared to stop within vision." Dwarf signals usually do not have train stops, that is, the automatic trip arm that enforces an indication of stop upon a train, although their indications are defined to be absolute. They are always interlocking-controlled; the mere presence of other trains may or may not affect the aspect of a dwarf signal. The indications and aspects are the same on all divisions. The newest interlockings on revenue trackage rarely employ dwarf signals (although sometimes situations of limited physical clearance mandate their use).

Dwarf Signal aspects and indications

Stop and stay

Stop and stay

Proceed with caution prepared to stop within vision expecting to find track occupied

Page Credits

Descriptions and graphics on this page (unless otherwise noted) are Copyright © 1997-2002 Bernard S. Greenberg (contact).