Judging from the way Albert Hammond sings this, I don't think it was intended to be a children's song. Allmusic.com has several sound samples by Albert Hammond and by The King's Singers, who, of course, do a very embellished version. I pieced together this much. I don't know if it's complete, and I don't even know if I got the verses in the right order:

I'M A TRAIN
(Albert Hammond, Mike Hazelwood)

VERSE: Look at me. I'm goin' somewhere.
I'm a train. I'm a train. I'm a chicka-train, yeah.
Look at me. I'm goin' somewhere.
I'm a train. I'm a train. I'm a chicka-train, yeah, baby.

CHORUS: Hard day, yes, it has been a
Hard day. Yes, it has been a
Hard day, yes, it has.
I'm a train. I'm a chicka-train.
I'm a chicka-train. I'm a train.
I'm a chicka-train, chicka-train, yeah.

VERSE: When life gets long and hard, (?)
I'm a train. I'm a train. I'm a chicka-train, yeah.
Goin' down to the breaker's (brakeman's?) yard.
I'm a train. I'm a train. I'm a chicka-train, yeah, baby.

[Mudcatters please note that I've refreshed a very old thread.]