It's not just about power to weight ratio. A Eurostar has about 22 / hp to the ton and can do 186 mph. Even the 1.8 VTEC Honda Civic with perhaps 100 bhp to the ton couldn't get near that speed.
Once the speed is high enough that the v-squared component of kinetic energy dominates, the power to weight ratio is almost everything, if the power can actually be delivered (it can't be delivered past a certain speed in the case of the car). The acceleration is strictly limited by the rate of energy gain divided by the mass (i.e. the power to weight ratio). The Civic simply isn't geared to deliver its peak power at speeds above 100 mph. The electric traction package on the train should be delivering close to max power all the way up to designed Vmax.
v-squared is pretty much the part of physics which causes the dramatic increase in required traction power needed as you go to higher and higher speeds (e.g. compare 75 mph, 100 mph, 125 mph, and 140 mph trains). Aerodynamics obviously does also play a significant part at higher speeds (and drag is also based on v-squared), so some of the power is lost to drag while the remainder goes to increasing kinetic energy.
E.g. for ECML passenger expresses, which should be vaguely comparable in overall weight over the years (and just going to engine spec rather than at rail, for simplicity):
- Deltic (100mph): 3300 hp
- HST (125mph): 4500 hp
- Electra (140mph): 6480 hp
