Final answer:
Without specific context, the rule number for Equipment Left on Main Track typically falls under rule 6.20, but the exact letter designation (A, B, C, or D) requires the context of a particular railway's operating rules.
Step-by-step explanation:
The rule number for Equipment Left on Main Track according to most standard railway operating rulebooks would be 6.20, with sub-definitions like A, B, C, and D providing further specific details. However, without the context of a specific railway's operating rules, it's not possible to determine the exact letter associated with a rule about equipment left on the main track. Rules such as 6.20A, 6.20B, 6.20C, and 6.20D could all potentially refer to various operational situations surrounding equipment on the tracks, such as unattended equipment, equipment fouling a track, or procedures for securing equipment.
It is advisable to check the specific rulebook pertaining to the railway in question for the exact rule number. Railroading has a complex set of rules and safety procedures, each meticulously documented to ensure the safety of rail operations and the prevention of accidents.