Sang this song in the Brownies in the fifties. The Railway verse went:-

Sambo joined the railways, for him there was no hope, He tried to scrub the railway lines with a bar of Sunlight soap. Along came an express engine, thundering down the track. Sambo rolled his shirtsleeves up and tried to push it back. BANG! etc

The chorus ended:- On his tombstone you will see, "Sambo the Brave and Free"
I might add we never thought of Sambo as a black man, more as a young white spotty apprentice type!