Final answer:
The maximum speed of a subway train over a crossover without a speed limiting sign depends on specific regulations, but it is typically around 15-30 mph.
Step-by-step explanation:
The maximum speed a subway train may move over a crossover, without a speed limiting sign displayed, varies depending on the specific regulations in place for that particular subway system. However, as an example, in some subway systems in the United States, the maximum speed limit for a subway train over a crossover without a speed limiting sign is typically around 15-30 mph.