The bus was travelling much faster than usual when it went through the bridge.
The bus travelled much faster than usual when it was going through the bridge.
Are there any subtle differences between the two sentences above?
"Was traveling" and "was going" treat the actions as processes, but "traveled" and "went" treat the actions as events. There is usually not much difference in meaning, but one might be preferred over the other in a particular context. For example an accident report, where the bus crossed the bridge and then had an accident as a result of its speed, would be more likely to use the first sentence. A report on a ticket for speeding on the bridge would be more likely to use the second.
When two things happened in the past and one of them was “continuous” in nature and the other was “short and sudden” and happened some time during the “continuous” action the “longer” one is in the past continuous tense and the “short” one is in the simple past.
Example: I was watching TV when the doorbell rang. I was waiting for a bus when I ran into my old friend.
The “bus & bridge” example is more natural when the “longer” action is continuous and the “short” one is in the simple past. It is logical to assume that the bus was probably “traveling” along its decided route when - at some point - it crossed the bridge and not the other way around.