For example, if the line between Royston and Cambridge is closed on a Saturday, the typical service remaining between London and Royston (for a rail replacement bus to Cambridge) on the East Coast Main Line will become hourly on each of Kings Cross - Royston non-stop, Brighton - Royston (fast between Finsbury Park and Stevenage), and Kings Cross - Royston stopping, where the normal service frequency is half-hourly on each of the above.
There is very long experience of doing train/bus interchange at Royston, going back to when Royston-Cambridge was electrified in the 1980s.
But regarding Royston, it's only 2 platforms rather than the multiple platforms at Cambridge
In this case Royston Station has only two bi-directional platforms, this limits the number of trains that can reliably terminate and start here.
Royston does have two platforms signalled for reversing trains, dating back to when electric trains terminated there 1978-88. But when the signalling was installed current rules on overlaps did not apply. Now, if an up train is departing from platform 2, down trains have to wait at Litlington, more than a mile away, making for long reoccupation times.
The platforms at Royston are narrow, especially under the Old North Road bridge, which is part way along both platforms. This means that long dwell times are also needed, especially bearing in mind that boarding passengers are in crowds of 50 coming off buses not a steady trickle.
Furthermore, the buses can only arrive/depart on the down side car park, the road access on the up side is narrow, twisty and hilly, thus not suitable for buses.
And currently there is a further complication: Royston has no footbridge. The footbridge installed before electrification has been closed for a while, having been declared unsafe, and the new footbridge, which will give Royston lifts for the first time, is not complete yet. So passengers arriving or departing from platform 1 have to use the Old North Road bridge to get from and to the buses in the down side car park.
In all of the last 30 or more years, I can't remember Royston ever reversing more than 4tph during engineering work, there simply isn't the capacity. Before Thameslink, when there were only 4tph anyway, and 3tph on Sundays, that wasn't a problem. But since Thameslink started in 2018 it has been impossible to run the full service when trains are terminating at Royston.
Despite these drawbacks, Royston is still the best place to do the train/bus interchange, because it is the only station on the branch with a big enough car park to handle the buses.
Sometimes it will be resourcing that means the reduction has to happen, in this case Cambridge provides a lot of trains for service as well as a lot of train crew.
This is a further complication since Thameslink started, in the old days a lot of the trains running in and out of Royston during engineering work would have been crewed by Kings Cross and Hitchin drivers, and I'm guessing that doesn't happen now.
Although why Great Northern/Thameslink don't put extra stops at the likes of Welwyn North & Knebworth isn't ideal
There is a simple answer to that, which is that the ECML timetable is a very delicate flower, especially getting trains through the 2 track section at Welwyn. During engineering work, the trains still have to follow their booked paths on the ECML so that they don't disrupt other services. There are few better ways of wrecking the ECML timetable than out of course stops at Welwyn North!
Who is there to 'let them off'? Sometimes, fewer people will travel when there is engineering work, so the service provision doesn't need to be maintained. For example, in the Royston example, if the line into Liverpool Street is still available, Cambridge passengers will go that way.
Unless the line into Liverpool Street is also closed due to the remodelling works around the Cambridge South station site.
There will be lots of blocks of both routes between now and when Cambridge South opens in 2025, including no trains between Christmas Eve 2023 and 2 January 2024.
