In the old days,the motive force for the train was (steam) locomotives, these weighed hundreds of tons (150+ for main line engines) and the weight was mostly distributed over the 6 (frequently) or 8 or even 10 driving wheels such that it was easy to obtain a distributed weight wheel load of 30+ tons at the rail, this load was great for traction through adhesion (and squashing pennies!).
The modern light build nature of the trains today have nothing like that wheel loading (frequently 9 tons ish) as the motive force is spread over 4 wheels of each carriage of the entire train carriage set (and makes then useless at squashing pennies!).
This lighter loading is a deliberate design to give greater life to the track the trains run on and make trains easier to operate.
Lighter wheel loads mean that you cannot effectively use a sand drop like they used to, but more importantly the train cannot STOP quickly (when required) under poor adhesion (less weight on the wheel and now greater braking forces because of disc brakes), the wheels just skid along the rails - and this is why the speeds are reduced when leaves fall.
We never should have dumped the steam locomotives!