how come there are no guard rails on railroad bridges

because they do not have enough friction

There ARE guard rails on bridges but not the kind you're thinking of.
If u look, u will see inside the regular running rails is a second set of rails, called a ''guard rail'' that is designed to catch a wheel that may be derailed & guide it right alongside the rail, this hopefully keeps the car where it belongs & upright.
It is far more important for the railroad to prevent a derailment than to try & contain it.
Of course a guard rail like u see along highways is possible but it would be horrendously expensive & heavy so the bridge would have to be much much larger & stronger.
It would not be cost effective, the railroad would have to divert funds form other parts of track maintenance & just doesnt make sense from a business standpoint.
And if the track is properly maintained, derailments r very rare on bridges.
Good question, a star for asking.

Guard rails would do absolutely nothing to stop a derailing train.waaay too much weight from the train.

Probably because if they were there that if the train hit it the train might still roll off, but ask Jango, he will know.

Next time you are on a bridge TAke a look between the tracks there is a second set of tracks, or rails. They are there for one reason, to keep the train straight and this way it will not fall off the bridge or ride off the tracks.

Trains are designed to stay on the rails and each carriage can weigh over 60 tons depending what they are carrying and there is no metal guard rail available that can stop a 50 car train weighing more than 3000 ton traveling over 80mph.

Probably impractical. Locomotives, by themselves, often weigh over 150 tons each. Add a couple dozen loaded box cars, gondolas, or whatever, and that would be a lot of moving mass to have to stop.

Very impractical. Train is too heavy and the best rails to keep it on the bridge are the ones it is riding on.