while i was on the steam train tour our guide told us that the locomotives don't have air brakes, they rely on the johnson bar to stop and the cars have brakes. how come? (i forgot to ask the guide why) if the locomotives don't have brakes, why do they have the air compressor?

The air compressor on the locomotive supplies air pressure to operate the brakes on the cars in the train.

I'm not sure about Walt Disney World but at Disneyland, the engines have train brakes that apply to the entire train but they do not have engine brakes that apply to the locomotive itself only.

According to the tour when I took it... They use the coaches to stop the train because of the experience for the passenger. If the engine "slammed on brakes", passengers would be thrown forward. With the coaches doing the breaking, passengers are pulled back against the bench during quick stops.

Well not quite, you can't break the laws of physics at those speeds, conservation of momentum says you will always be thrown forward where ever the breaks are. The ride would be harsher in the rear cars because of couple slack, the first car would have little slack and would be pushing on the locomotive after a few inches of travel but 5 cars back you get over a foot of travel and it would be like hitting a wall at low speed.

that makes sense, no wonder the cars screeched when the train was stopping

I don't believe the WDW engines ever did have brakes--probably no need down in the Yucatan. This is the way locomotives were stopped back before the advent of air brakes--close the throttle, put the Johnson Bar in reverse, and crack open the throttle a little.

Yes, the Disneyland engines have brakes, and both engine and train brakes are controlled from one brake staand, unlike most other steam engines, were there is an independent brake (controls only the engine brake) and a train brake--which controls the brakes on the cars. The two brakes are used in conjunction with one another to keep the cars from slamming into the engine, and to keep the slack taught on the train.