Add ability to filter by sub-line (GL B/C/D/E & RL Ashmont/Braintree)

[giff-h]

This would be very helpful for when there's several trains bunched up in one rendered location, and you have to keep selecting the location to scroll through the trains, and it doesn't always scroll through all of them.

Alternatively/simultaneously, it would be nice to color code the sub-lines and expand a bunch of overlapping trains when one is selected.

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had activity within 180 days.