What is the approximate voltage on the signal main?

The approximate voltage on the signal main for MTA Transit systems is typically 24 volts DC or 12 volts DC, which is used for low-power signaling functions on transit systems. However, when discussing the given options, the closest relevant voltage for electrical components in transit systems could refer to circuit voltage standards.

The correct choice here, 120 volts AC, is a common voltage level used for power distribution and electric systems in general, and it is often applicable in contexts where auxiliary power might be involved, such as lighting or control circuits within stations or maintenance facilities.

While 60 volts AC is sometimes used in larger rail systems for certain signaling tasks, it is not as standard as 120 volts AC in the broader context of transit systems. The higher voltages of 240 volts AC and 600 volts AC are typically used for heavier power applications and are not as closely associated with the general signal main operation.

Thus, understanding that 120 volts AC serves many maintenance and operational needs within electrical systems makes it the most appropriate choice given the options.