[bluGill]

300kph is the limit because aerodynamics make that about the best compromise on the effeciency cury. higher speeds are completely possibly - but air planes running with much less atmospheric drag start to become the better option.

of course the above is all about compromise and you can emphasize whatever numbers you want to get different results.

Edit: it is often a good idea to have everything capable of faster speeds - say 350km/h. You don't normally want to use those speeds, but if a train gets delayed (as happens) you can use that extra speed to make up time. Just don't let this become a normal thing.

[Pet_Ant]

What about if they added “wings” to trains? That could generate some lift reducing the effective weight is my shower thought.

No idea how much the wings would add versus the lift help.

[NuclearPM]

The friction is almost entirely from drag/air resistance, not from the resistance of the rails.

[bluGill]

the losses from weight are linear with speed - at high speed completely dwarfed by losses from pushing air out of the way which is quadradic with speed.

the wings on race cars are poited down - they increase weight to keep the car on the ground at the expense of more drag, which they overcome with a bigger engine (and more fuel use)