With realistic physics, one can disable slowing down in slopes, but not in curves. With original physics, it is the opposite.
How do I get trains that won't slow down in either slopes or curves ?
As for why I want that... I like to build large hi-speed networks, with cyclotrons at the entry points and crossings. But then, at crossings, one has to go over or under some tracks, which reduces train speeds. Dealing with this bloats the crossings.
With realistic physics, cyclotrons must be very large, diminishing their efficiency. Dealing with this leads to oversized and complex entry points, and crossings, too.