One subway car weights about 30 tons empty, breaking from 15 m/s (~60 kmh) to a full stop it converts almost all its kinetic energy into heat so each car produces 15^2*30000/2 ~ 3MJ at every stop.
An interesting thing is that many subway systems recover some of the kinetic energy by converting it to something more useful than heat.
Some subways, for instance London's, are built with uphill slopes prior to stations, so that part of the kinetic energy is converted into potential energy.
Many subways use regenerative electrical braking for part of coming to a stop, i.e. the kinetic energy is partially converted to electric energy which is used by other trains on the network. Some subways increase the efficiency of that by having storage, for instance a large battery, in stations.
