To @M1dn8 @gekberlin:
As @derhuerst mentions, this feature can be implemented, but it is a bit of work… We can try this. If you have any programming skills, your contribution is more than welcome :)
To @derhuerst:
Do you think, that some other possible future services that use your API will likely use this feature? If so, I think it should be done in the API. If not, I can just throw away the entries that are not used. But as you said, determining the direction could be a bit tricky. So one would need to know the next station on the line, huh?