All depends on whether the rights holder even cares. It could be the folks creating Northern Pacific Air talked to BNSF and they were okay with it. If they didn't and BNSF sued , BNSF would need to make a case that the airline is similar enough to the train service that it would be harmed. Given that the Northern Pacific hasn't existed since 1970 and this is an airline, would be a very hard case to press.
On the other hand, the owner of the 4 seasons restaurant in NYC once sued a hamburger place out west for using the name "4 seasons", hopefully the judge threw it out (I don't recall), was patently ridiculous.
