I assume sidings can still speed up trips.
An extreme example: if you had a 100km section of single track, in order to have two trains pass each other you'd have to hold a train at the end and wait the time it takes the other train to go the whole 100km before the first train starts. If you put a passing spot in at the 50km mark, both trains can start and then pass in the middle. I have no idea the actual situation here but it seems plausible situations like this exist and will speed up trips.
An extreme example: if you had a 100km section of single track, in order to have two trains pass each other you'd have to hold a train at the end and wait the time it takes the other train to go the whole 100km before the first train starts. If you put a passing spot in at the 50km mark, both trains can start and then pass in the middle. I have no idea the actual situation here but it seems plausible situations like this exist and will speed up trips.