Yeah train wifi is generally terrible. Basically the only real solution is 100% 5G coverage along all train tracks or starlink. This is basically a global issue because line-of-sight for trains is a hard problem.
I've used some of those WiFi services at near 200mph, so I guess it'll depend on the 5G, LTE or HSPA frequencies it connects to. They're never great, though, although I guess that's also related to tunnels and the quantity of people aboard.