The proper place to post this question would be the Classic TOY Trains forum. However, I can identify what it is you have. Your train is a model of the Union Pacific M-10,005 streamliner made by Marx. Louis Marx and Co. was a prolific toymaker (at one time the world’s largest) who produced many toy trains. Your train was manufactured in the 1930’s and runs on O gauge track. As far as value goes, the Greenberg Marx priceguide lists the following:
Locomotive (may be either electric or wind-up): $40 good condition, $60 excellent condition
Passenger cars (may be named “Los Angeles”, “Omaha” or “Denver”): $15 good, $30 excellent a piece
Observation car named “Squaw Bonnet”: $20 good, $30 excellent
Car lettered “M10005-RPO” and “REA/RPO-UP”: $30 good, $60 excellent (the majority of sets didn’t include this car, hence its higher value than the others)
These trains were made in a valiety of colours, but since you mention green, I’m assuming that it is a pale cream colour with green roofs. The values given are for a train in these colours. Your comment “it may have been painted” worries me, though. If someone has indeed repainted this train at some point, the collectable value is almost completely lost and your train would be worth less than half of the values given for items in good condition. Collectors are looking for items in original condition and a repainted set is generally seen as either a parts source or else a candidate for customizing or some other project.