The usual signal bulb is 10 Volt and either 18 or 25 Watt. Signal lamp power is nominally 12V to allow for wiring voltage drop and adjustment to provide the required 10V at the bulb.
Primary batteries are sometimes used, in which case approach lighting control is usually required to get reasonable battery life.
More common is that 12V is provided by a transformer from an AC power source at the signal location or via trackside line wires. A rectifier which float charges a secondary battery may be provided for backup if the AC fails.
In some remote locations where AC power is not available and primary batteries were formerly used, solar power panels are now used to charge secondary batteries.