Hello, and thanks for reporting these!
> trains in transit to or from Lowell Station (after or before North
Billerica) always have "null" and "unknown" as the destination
Could you clarify where exactly you're seeing this? For example, if you're using the V3 API, which resource/attribute. We checked trip 1307 today on its way from North Billerica to Lowell, and didn't see anything unusual in either the predictions or vehicle data.
> the 4pm service from North Station and its return, never seem to show
This is likely due to a faulty tracking unit in the train that runs this trip. Unfortunately, the current tracking system for the Commuter Rail is no longer supported and in the process of being replaced, so we can't provide an estimate of when this will be fixed. In general, for Commuter Rail (and bus), we recommend falling back on schedules when real-time data is not available, since tracking issues occasionally happen during normal operations.