First Great Western Trains say that one of their drivers suffered serious cuts and bruises to his face after an object the size of a brick was thrown from a bridge.
The 'rock' smashed through the driver's window as the train made its way from Romsey to Salisbury.
The Portsmouth to Cardiff service was delayed by an hour. A doctor on board the train treated the injured driver and a South West Trains driver stepped in to take the locamotive to Salisbury.
More top news
Forecast for tonight and tomorrow in the west
Forecast for tonight and tomorrow in the east
The private school teacher pleaded guilty to 11 incidents dating back to the 1970s and 1980s.