Chesapeake and Ohio theme from the 1989 rulebook on Railroad Signals of the US website based on SpDrS60 theme. This rulebook is a superset of the earlier 1957 and 1969 rulebooks so can be used for earlier installations by ignoring the flashing aspect but not all the variations of aspect found in the 1957 rulebook are present.

This is an interesting system in that there is no lunar and dark on the lower head has distinct meaning different from red, green and yellow. Stop - rule 292 red/red/(dark) - is not available on a single head mast, the most restrictive single head aspect being stop and proceed.

The following signal masts are supported:-

Prefix	Mast type
	twin head mast automatic
sa_	Single head mast standard head aspect numbers
s_	Single head mast unified C&O aspect numbers
d_	Twin head mast unified C&O aspect numbers
t_	Triple head mast unified C&O aspect numbers

Automatic aspect numbers
Aspect	Rule	
-r	R292	red / red
-y	R285	yellow / red
-g	R281	green / red
-w	R290	red / yellow
-b	-	unlit


Standard head aspect numbers
-0	R291	red
-1	R285	yellow
-2	R281	green
-8	-	unlit

Unified C&O aspect numbers
-0	R291	red / red / (dark)
-1	R292	red / (dark)
-2	R290	red / yellow / (dark)
-3	R288	red / yellow / red
-4	R287	red / red / green
-5	R286	red / yellow / yellow
-6	R285	yellow / red / (dark)
-7	R285	yellow / dark
-8	R284	yellow / yellow / (dark)
-9	R283b	red / yellow / green
-10	R283a	red / yellow / flashing green
-11	R283	red / green / (dark)
-12	R282	yellow / green / (dark)
-13	R281	green / red / (dark)
-14	R281	green / (dark)
-15	-	unlit
