Trespassing is illegal and dangerous especially when inside the tunnel with a train! If you go inside and see a light run and hide! that is unless of course its the portal, then you don't have to run nor hide. Trains burn diesel fuel and produce among other things carbon monoxide and deafening amounts of noise! Trains also have people in them and people have eyes used for seeing things.. Like trespassers!
Just be careful, use your head, and stay safe.