Train speed depends on several factors-
1) the route you are travelling on may be on a downgrade, therefore increasing the speed of the train as it is being pulled down by gravity
2) the train itself may be faster
as said openbve simulates them realistically...if you want to modify the actual train to be faster, you'd have to edit the Train.dat file using train editor (or manually if you know how to)
if you want to edit the route to make every train go faster, change the @pitch or .pitch value (the lower the negative number, the steeper the hill)