I remember reading a story of a runaway teen, trying to escape social services. I remember he was on a train and ran into a group of active or retired military and stuck to them and later got help. I thought it was located in Australia, and thought it was one of Ernest Bywater's. But upon reading all of his, I couldn't find it. I know it's not a lot to go on, but would appreciate help.
No trains in Runaway. And the only ex-military is the cousin he found and lives with as he grows up.
You might have two of Ernest combined.
Runnaway A 9 year old boy's parents are killed by terrorist. He goes to live with his great grandfather until he's 16 when his great grandfather dies of cancer. His parents raise him to be free thinking and independent. His great grandfather adds to the training. David wants to stay living in the house he helped to build, but the welfare authorities won't allow it. So his great grandfather helps him to be true to himself when he runs away to stay free, and he finds a new life
Survivor
A 10 year old boy survives a car crash where his family is killed. He goes to live on his maternal grandfather's ranch on the Texas / Mexican border with some of his retired Army buddies. But life there has some serious issues due to drug smugglers being angry about having their shipments destroyed by those at the ranch. Pat grows up fast while learning to be quick and accurate with a gun.
There a second part to Survivor too.
But again, no trains.
BTW, Ernest is sorely missed. I was really hoping for a sequel to 'Tinker, Taylor.'