Thanks for your suggestions - a few comments to each section:

Vehicle restrictions - I have really struggled to re-implement this, because the API allows access to all vehicles and doesn't tell you what restrictions are in place (if any) so I have had to use some different ways to figure out the restrictions and they are not completely robust. I may put in an override in the new config menu.

Depot placement was a big problem early on, it is amazing how often it would end up in completely the wrong place. There is logic that builds the depot along the route as a backup, but this is unreliable like if the line is entirely over water.

Calculating the cost is possible though it requires going through a full build it could be something I implement later as its not totally trivial as it would involve some callbacks and sending the data back to the UI thread.

With line upgrades yes I wanted to have a "double track" option, to automatically do it. Now in the lines report it will suggest double track if the line only has 1 train - this is a performance optimisation so that it doesn't do the work of figuring out if it really is double track until you actually click the "execute" button.