What is the correct word for a "substitute bus" in English? is it correct? it sounds somewhat strange to me.
"Substitute bus" is correct. Another term that is also used is "replacement bus".
In England you would be more likely to see "replacement bus", usually in the context of replacing a train.
The meaning of "substitute bus" is comprehensible but it does sound a bit strange to my ears too.
But do remember that the English used in England differs from the English used in the US, Australia, New Zealand,the Caribbean, India and the many nations around the world that use English as a first or second language,Edited: 05 August 2018, 16:56
In the US, I have seen 'shuttle bus' as a replacement for commuter trains and subways in the Philadelphia area, and 'alternate transportation' for intercity rail service. 'Substitute bus' doesn't sound good, but is clearer than the other phrases.