The electric trains to run require electic power 25.000 volts supplied from cables hanging above the train,the power is fed into the electric traction motors,which powers the wheels, these traction motors require cooling air to stop overheating.
It appears that snow has been sucked into thes motors, and then frozen,on entering the tunnel that actually requires water pipes to cool it, has melted the snow,this in turn has shorted out the traction motors.
If a train breaks down in the tunnel the one behind can be used to push the broken one out, or tow it out.
Diesel trains can operate in the tunnel,but due to the fumes from the diesel engines,are used only on limited occasions.
AS for useing other older trains to pull them out,there may well be coupling incompatibility between the train and the loco.
As for being trapped on the train, just use the emergency door realease, and walk down the walkway, all lit up, and use the cross passage to enter the service tunnel,where there will be comunication with the control centre,and the emergency services.