I wish I could help you out, but sadly I'm also in the dark about 'The Secret Railroad'. However, I'm part of an animation forum where people discuss and trade old cartoons. I'll drop a post there and see if anyone's got the inside track on your show. Will keep …

Heh kiddo, well ya gotta know that Curious George is based on old-school children's books. He was always a mute main character. They probably kept it in the show so kids can imagine what he's 'saying' through his actions. Makes the lil' ones think more IMHO.