Peter's Railway Holiday at Lunan Bay by Christopher G. C. Vine
Peter is on holiday with his family in Scotland when Grandpa learns of a spectacular train crash. It happened nearby so they go exploring to find the exact spot. To add to the adventure they discover some evidence still lying on the ground - It's a true story where a crazy mistake led to disaster, but luckily no-one was hurt. To show how easy it is to make a blunder like this, Grandpa tells another story which ends with a Bang!