Marx 1420 Gateman, mid 50's. There was an illuminated version also.
Go to Robert Grossman's Parts for Marx site, http://www.trainpartsformarx.c...cart.htm#ACCESSORIES
Part number 265 is the replacement arm.
I cannot tell from the photo if the weighted counterbalance is broken, if so, e-mail him about a replacement. I think he has now found someone to make them again.
Steve "Papa" Eastman