It's perfectly feasible to have far quicker diesel trains than is common in the US (see the British High Speed Train, from the 1970s, with a top speed in servic

It's perfectly feasible to have far quicker diesel trains than is common in the US (see the British High Speed Train, from the 1970s, with a top speed in service of 125mph, or the ICE-TD from a decade ago, again 125mph). Of course, you then need to have signalling for that line-speed and have pathings that allow it.