First off, more than 19 volts is over the limit for the max voltage specified for most Lionel engines. The LC manual I have specified 18V and doesn't mention the maximum, but since all the TMCC and Legacy stuff specifies 19VAC maximum, I'd say it's safe to assume that anything over that is probably not good for the engine's long term health.
This is also a sample of one, I doubt with no speed control that all LC engines and/or sets will share the same speed graph.