150s can go 30mph faster than 142s on the same track
 

I recently discovered that a 20mph speed limit between Northwich and Greenbank in Cheshire is only in place because Pacer 142s only run on two axles so cannot safely go over 20mph over a long viaduct. However, if Pacers were banned from the line then the speed limit could be raised to 50mph.

Apparently there can't be one speed limit for one type of train and another for a different type.

On the line between Birmingham and Derby the linespeen is 125mph for HST's and Voyagers and 90mph for other trains so it is possible to have different linesppends.

Well maybe it's because Northern Rail run all the passenger trains on the lines and they all have a 75mph maximum speed, so it could cause confusion for the drivers.

Between Birmingham and Derby I would imagine trains owned by Virgin are the only one's capable of travelling at 125mph.