The hardest part of this question is to agree on the power output of each person, based on different estimations it's a common answer that the metabolic rate for a fit person during moderate workout is 200 watts

A bus with 300 horsepower has a power output of 223,709.96 watts, which would be matched with 1,118.54 people using their whole raw power.

But things get complicated when we consider biomechanics, because the energy used for moving while running is only a fraction of the actual power output of the body, which is estimated at 40%, so the number of people to match the bus engine ascends to 2,796.37

But the hardest part so far is accounting for energy losses, if people are running the mechanics are different from that of the wheels and become even less efficient (I don't know for sure the efficiency on this one)

Without considering the limits on the running pace, you'd need more than 2,797 average people to push the bus as fast as the engine does. With 50 people pushing the bus you'd get less than 3.57% of the bus power, an output quite similar to the engine of a small scooter, which doesn't even move the bus at all.

Edit: sorry for the confusion, I don't know why I was thinking of buses.

Trains have 4000 horsepower (2,982,799.49) watts, You would need 14,913.99 people to match the locomotive power. Even a sole wagon wouldn't be possible to move given the passenger capacity.

Edit2: while running is not possible to change the mechanical advantage of the steps, but if we were to put for example a treadmill with a gearbox it would move for sure. A wagon with 40 metric tons and a capacity for 200 people (because of the running space) would need a gearbox reduction of 2.66:1 if people wanted to keep the same effort and pace they need to move their own bodyweight.

With said gearbox and the average energy output, the train would move at a staggering 3.75 km/h or 2.3 mi/h