There are varying speed limits along the rails, just as there are on highways. This train’s rails have up to 70mp limits; at the location of the crash, it was 50mph.
Along the same route there is a speed train (Acela), and new ones coming next year that will allow speed up to 160 mph. That’s not the standard train, obviously.
First off, let's define the FRA's speed limits for trains. Outside of Amtrak's Northeast Corridor, which is a special case, freight trains are restricted to a top speed of 70 mph, and passenger trains are restricted to a top speed of 79 mph. There are specialized rights of way where passenger trains are allowed a top speed on 110 mph.
On the Northeast Corridor, the top speed is 150 mph, but there is only a short stretch in New England where that speed is permissible. The rest of the Corridor has a top speed of 135 mph, but that is not everywhere, just certain stretches.
The place where the accident occurred has a speed reduction from 80 mph to 50 mph due to the curve at Frankford Junction. The clown that was driving the train was moving at 106 mph through the 80 mph zone, and he hit the 50 mph zone at 102 mph.
In 1943 the same thing happened when the Pennsylvania Railroad's hotshot Congressional Limited jumped the tracks at that exact spot due to operator error. That time 79 people were killed. There were fewer people killed this time because our passenger cars are built more strongly.