I have a trane closed unit in the ceiling and it keep tripping the breaker. I can't even turn on the breaker cause it will trip and no stay on. Could it be that the transformer is not working in 24 volts and could be faulty, or could it be something else?

something else,,,,be careful a dead short is nothing to mess with,,,,,you could cause a fire,,,,could be as simple as a bad breaker

Is this something that just started or did you do some work on the furnace, such as clean the filter.

With the breaker off go up and disconect the white wire where the cord enters the furnace, now turn the breaker on, if it trips you have a short in the line going to the furnace.

If it doesn't break it could be a shorted transformer.

If you have done something with the wiring is it possible that you changed the pollarity, they are polarithy sensitive but I am not sure that would trip a breaker.