The Baltimore and Ohio railroad used a unique color-position signal system that combined the use of colored lights with the semaphore simulation of the PRR / N&W position light type signals. Below is the master plan of a B&O signal.

The main signal head contains the color-position lights that show the equivalent of clear, approach, stop, or restricting. The following diagram shows the appearance of these signals for each block condition.

In addition to the main signal head there can be up to six auxillary lights above or below the main head and offset to the left or right of the main head. Very few signals have all six of the auxillary lights.

• Most block signals only have one auxillary light, the center top one.
• Interlocking signals, and those signals approaching interlockings, usually have two or more of the auxillary lights but not all of them.
• On signals equipped with multiple auxillary lights only one auxillary light will be illuminated at any one time.

The auxillary lights may seem to be arbitrary and confusing but they are not. The diagram below shows the master logic of the design.

1. If any one of the three auxillary lights in the top row are present and illuminated that allows normal speed through the block.
2. If any one of the three auxillary ights on the bottom row is present and illuminated that requires Medium Speed at this signal and the first word of the signal name will be Medium.
3. If either of the auxillary lights to the left of center are present and one illuminated that requires the train to be at Medium Speed at the next signal and the signal name will have Approach Medium in it.
4. If either of the auxillary lights to the the right of center are present and one illuminated that requires the train to be at Slow Speed at the next signal and the name will have Approach Slow in it.
5. If none of the auxillary lights are present, or are present but not illuminated, that requires Slow Speed at this signal and the first word of the signal name will be Slow.

As seen in other signaling systems certain indications can be upgraded, bumped up to a less restricting aspect, by the flashing of lights.

• The flashing of the top left auxillary light upgrades it from an Approach Medium to an Approach Limited.
• The flashing of the bottom center auxillary light upgrades it from a Medium to a Limited.
• The flashing of the two vertical green lights is a special case which upgrades it from a Slow Approach Slow to a Slow Clear.

The main color-position head displays the block occupancy information. Occupied in this context means there may be a train, engine, cars or a hand throw switch not properly lined or a broken rail within the block.

• If the main head is Green (vertical) then this block is clear and the train will not have to stop at the next signal.
• If the main head is Yellow (45 diagonal right) then this block is clear but the train must be prepared to stop at the next signal. The last word of this signal name will be "Approach".
• If the main head is Red (horizontal) or Lunar White (45 diagonal left) then this block may be occupied. and/or hand throw switches may not be lined for the mainline.

Now that we have the basic logic of the system the B&O color-position light signal rules shown below should not seem so arbitrary.

Rule	Aspect	Name	Indication
281		Clear	Proceed
285		Approach
281B		Approach Limited
281C		Limited Clear
281D		Limited Approach
282		Approach Medium
283		Medium Clear
283A		Medium Approach Medium
283B		Medium Approach Slow
286		Medium Approach
284		Approach Slow
287		Slow Clear	Rule 287 and 287A (below) do not follow the B&O logic. A green main head alone with no auxillary lights lit should be a Slow Clear. But if it were then there would be no way to display a Slow Approach Slow. So the B&O required that the green signal alone be flashing to display Slow Clear, and if not flashing it was a Slow Approach Slow.
287A		Slow Approach Slow	*See note above.
288		Slow Approach
290		Restricting
291		Stop and Proceed
292		Stop

Notes:

Created 01-15-2006
Updated 11-06-2006