At first,
in North American railways, when talking about speeds, they don't state
the speed itself (e.g. 48 km/h or 30 Mph) but they give it names. That
allows for greater flexibility, because e.g. different railways may use
different speeds on shunting tracks, points or similar.
Speaking about speeds, we have clear
(i.e. maximum speed), medium
and slow.
The signals were built to reflect these three speeds.
Later, and differently on different railways, two more speeds
were invented, these being limited
(faster than medium, but less than maximum), and
restricting, which usually is slower than of the same as slow, but you ride on
sight, i.e. you must be prepared to stop within visibility distance because the
track may not be clear. I will not cover those here, though.
As a train has a long braking distance, a speed restriction or stop
must be indicated at least one signal in advance. In Europe, we use
distant signals for that purpose, or we use multiple-block signals. See
my pages on German signalling systems
for details.
The North American colour light signals usually
indicate the speed you may travel at after this signal as well as show
which indication the next signal will display. In Europe, we would call
that a multiple-block signal.
The various aspects (that is what the signal looks like, e.g.
red-over-green) and indications (that is what you must do, e.g. slow
down) have names so they can easily be referred to.
As example, when this signal allows medium speed and the next one would
indicated slow speed, the first signal would show
red-yellow-green, and it would be called "medium approach slow" in the
US. In Europe, we would say something like "80 km/h, expect 40 km/h"
and the Canadians would say "medium to slow".
So, the US and Canada have a slightly different terminology (the
Canadians are a reasonable crowd anyway and follow European logic a little closer. Not only in railway
terminology, they allow people to drink in public and ban guns just like us). Note that in a
name like "clear approach stop" US Americans drop the clear or stop, so
they'd call it just "approach", and "clear approach slow" becomes
"approach slow", and "medium approach stop" would be called "medium
approach".
Here's a small inexhaustive comparison table, but it should help you
with the logic.
| US | Canada | Europe |
| clear | clear | clear |
| stop | stop | stop |
| approach | clear to stop | expect stop |
| medium approach | medium to stop |
medium, expect clear |
| approach medium | clear to medium | medium, expect stop |
| medium approach slow | medium to slow | medium, expect slow |
| slow approach medium | slow to medium | slow, expect medium |
Historically, signals were semaphores, and later the night aspects became the prototype for the colour light signals. That is exactly the same development as e.g. in Germany. The US (like Germany in the early days of railways) followed the British system of not signalling speed but rather the route, i.e. a signal would indicate whether you would travel through points set straight or deflecting. The British accomplished that by having two signal arms above each other, the higher one would govern the straight (faster) route while the lower one indicated the deflecting (slower) route. A horizontal arm (at night, a red light) would mean stop while an inclined (or horizontal) arm (at night with a green light) would mean clear.
So you would have these aspects: red-red for stop, green-red for clear, red-green for slow. For comparison, I show the historic German aspects, too.
| clear | slow | stop | |
| US signals | |
|
|
| German signals | |
|
|
As you can see, the British / US philosophy is, that the upper arm governs the straight route while the lower arm governs the diverging route. If the points are set at diverging, then the straight route is impassable, and consequently the top arm is horizontal and the upper light is red.
On the contrary, in Germany, both arms are raised to indicate slow. So that is the main difference between European and American signalling:
So, to show three speeds, we have three heads. If all are red, it means that no route is passable. If one head is green, the corresponding route is passable. As example, red-green-red would indicate medium speed, while red-red-green means slow.
clear
with medium speed
Above I have stated that it is necessary to indicate in advance whether a signal is showing a less favourable aspect or even stop, so that the driver can begin slowing down before the next signals comes into sight. Talking of North American colour light signals, basically speaking, they use the same aspect to indicate clear, but use a yellow light instead of a green one. So the aspect
shows clear with medium speed at this signal, but the next signal will
show stop. So effectively a green light tells you that at least two
blocks are clear with the current speed, and a yellow light tells you
that the current route is clear for one block only.
But because of the long braking distance of trains, you must warn the driver not only of a stop ahead, but also if he has to slow down. So look at this aspect:
The yellow upper head tells you that the high speed route is clear for
one block only (i.e. you may pass this signal with maximum speed, but
the next signal's high speed head will be red). The green medium speed
head show you that the next signal's medium speed head will not be red.
So this aspect means 'clear to medium', or, as US Americans would put
it, '[clear] approach medium'.
Now we can figure out this aspect: It shows clear to slow (approach slow).
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