thomas and friends mini series

Do you mean the numbers printed on the bottom by the engine's name, like "H17A?" Because that's just a manufacturing date code (H = 2016; 17 = 17th week of the year (mid April)); I haven't heard of anyone doing that but I guess you could collect different versions, but if the factory has any consistency (ISO-9001...) there shouldn't be any difference week-to-week.

Or do you mean the numbers imprinted on the plastic itself in the interior of the train, like "CGM30 2129 11?" I'm not sure what those are for; I started to try to make a spreadsheet of my trains but stopped pretty quickly when I realized many engines shared the same values so there wasn't anything unique about them.

yes the printed number on the bottom by the engines name. thanks for clarifying that i only mention it cause the new batcave batman has different code the other batmans and so do 2016 duplicates and so on all mine are open cause my son plays with them and dont wont to rack my brain on keeping the code with release date together we just collect each theme no duplicates if possible