Train transformer starting voltage is a harder question than it necessarily needs to be–because it’s varied over the years. Part of that is because intent has changed over the years.
When trains were toys for boys, high speed had great value. Today, when there’s a significant element of the hobby that places great value on realism, low speed has great value.
Generally speaking, until the 1980s or perhaps even the 1990s, the starting voltage of an O or S gauge transformer was around 5-6 volts. Some motors could run at a lower voltage than that, but not all.
There are exceptions of course. A Lionel 1033, for example, can be wired to provide 5-20 volts or 0-11 volts depending on which posts you use. So it’s possible to give up high-end voltage to get a lower starting voltage if you like trains that creep slowly.
Modern transformers are much more likely to be able to deliver both a low starting voltage and a high maximum voltage. This accommodates hobbyists who want to run their trains the the conventional, old-fashioned way but want low speed and also accommodates hobbyists who use one of the many forms of command control, which generally runs at about 18 volts.
Modern transformers offer a lower starting voltage than their vintage counterparts, and some of them ramp up the voltage from zero to wherever the operator sets the throttle, to avoid jackrabbit-like starts.
Adding a voltmeter is a good way to reduce the guesswork.