And everyone would be killed.

Her hands grew very cold and trembled so that she could hardly hold the flag.

And then came the distant rumble and hum of the metals, and a puff of white steam showed far away along the stretch of line.
"Stand firm," said Peter, "and wave like mad! When it gets to that big furze bush step back, but go on waving! Don't stand ON the line, Bobbie!" The train came rattling along very, very fast.
"They don't see us! They won't see us! It's all no good!" cried Bobbie.
The two little flags on the line swayed as the nearing train shook and loosened the heaps of loose stones that held them up.

One of them slowly leaned over and fell on the line.

Bobbie jumped forward and caught it up, and waved it; her hands did not tremble now.
It seemed that the train came on as fast as ever.