Currently the third-party routing service we use doesn’t provide accurate rail distances for these trips, so we are using road distances for the moment.
In large cities with large underground/tram networks (eg London) these are likely to be fairly close to the actual distance and the error will be quite small. In places like Glasgow, there will be trips where it’s much shorter to drive directly across the city than it is to go round the edge on a circular underground line. Our calculations for these trips will therefore be slightly out.
If your organisation does a small amount of tube travel and a reasonable amount of flying, driving, and longer distance journeys then the error will be fairly small.
On the other hand if all you ever do is get the tube from one side of Glasgow to the other the errors may start to add up. If this is a problem for your organisation, you can override the suggested distance when you add the trip.