A single E-locomotive 3 will top out at 518 km/h but as soon as you start adding cargo wagons the speed drops to 324 km/h unless you have 3 engines per wagon.

Im also haveing this issue i have placed tons of the control units. i thought they would be getting more power but i guess not

Hi,
i see the same, but, not on all trains so i tryed to figure out what happen.
if there is one wagon that is NOT MK3 - you don't get max speed.

hope that helps ;)

Can confirm. Upgrading all wagons to MK3 makes train reach its maximum speed.
It looks like the MK1/2 wagons are simply not rated for these speeds. Probably old style bearings that pop at 350?