For the uninitiated, there are many different types of Thomas the Tank Engine trains to collect.  If you like to shop on eBay, you'll find plenty of the trains, both used and new.  This is one of the easiest places to find the Ertl trains and their special track, which are becoming scarce.  If you live in Europe, you'll probably still find new Ertl trains, still in their package at stores that carry the Thomas trains.

Take Along Thomas the Tank Engine trains are also easy to come by, and much less expensive than their wooden counterparts.  They can be found in single, double and triple packs.  They are hefty, but fit perfectly in the hands of your favorite toddler.  They're built to stand up to a lot of punishment, which may also be the reason they'll last long enough to be handed down generation to generation, or collected just for the fun of it.

Wooden Thomas the Tank Engine trains are probably the most popular Thomas trains to collect, due in part to their nostalgia.  They range in price brand new from $11 (USD) to about $32, again depending upon how many trains are in the package.  These are pretty easy to locate online and in your favorite toy stores. 

Battery Powered Thomas the Tank Engine trains are also easy to find at your favorite toy store.  They are slightly less expensive than their wooden counterparts and will require batteries to operate.  The types of batteries required range from AA to C, depending upon when they were made.  Many of the older battery operated Thomas trains require the C batteries, which unfortunately slows them down on the newer tracks, especially if there are lots of hills on the track.

Pull Back models of the Thomas trains were designed for smaller children.  All the child has to do is hold the train on the floor and pull it backwards.  The train will then race forward by itself for several feet.

Remote Control models of the Thomas trains include the Thomas train along with several cars, which clever children usually fill with their other favorite small toys.  This train requires several AA batteries in order to operate.  The remote control is easy for small hands to maneuver and difficult to destroy.

What you should collect should really depend upon what you like about the individual trains. Personally, I enjoy collecting the Ertl trains because there are some characters that haven't yet been made into the other versions yet. Many of these characters are from the books in the series. These trains are becoming valuable, so if you find them at a local garage sale, grab them up.

The wooden trains are very popular to collect, however they have one major drawback, at least in my own experience. The paint chips fairly easily, and any child who plays with them constantly will eventually wear the paint off in some areas, as well as add quite a few dents and dings into the wood. These trains can never be kept in a damp area either, as their paint will definitely peel off and the wood will warp. If you choose to collect this version, keep them in their original package- and buy a second for the kids to play with.  Keep on the look out for special or anniversary editions- those increase in value quickly because the numbers are limited.

Take Alongs are not as popular as a collectible, with the exception of their limited edition trains, usually found during the holidays. Collect these unusual ones, such as Christmas train and Easter train that come out each year (they're always different each year). Get them while they're hot, since there are few sent to each store.

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The Battery Powered trains are lots of fun to collect and their price makes it easy to amass the entire collection quickly. They should last for years, as long as the batteries are removed when not in use. Their value, unless they are discontinued, probably won't increase in the near future.