Yes. However, at some places, like inside some buildings, you have a good wifi but very bad mobile network reception.

In high speed trains the wifi might also be stabler than your phone because the system behind the wifi is designed to work with the high speed you are moving at.

My experience with train wifi is that the wifi signal is great but beyond the portal nothing else works.

I regularly take the train. I've had good experiences most of the times, and catastrophic experiences some of the times. It depends on the location, also. Some lines are better served.

This is not something you can rely on however. Take it as a bonus if it works.