As of now, there are four (or more) minor characters in the series: Diesel 10, Rocky, The Breakdown Crane, and The Drivers and Firemen.
1. Diesel 10 is a nice diesel who also helps small trains out of small wrecks with his claw. With the claw, he seems menacing, but is not. D10 was once evil, and even tried to kill Thomas. For some dumb reason, Sir Topham Hatt let him back on Sodor, assuming he would never be evil ever again. Lucky for Sodor, he wasn't. D10 can't stand it when his claw, Pinchy, hits him on the forehead.
2. Rocky is a very large crane who can help engines even out of the biggest messes. Everyone treats him like crap and chooses the Breakdown Crane over him because TBC was in the original series before HiT took over Thomas. Rocky is only chosen if TBC is unavailable, or there is some gross disgusting job no good person should ever undertake. Rocky can't stand the fact that he was created by HiT.
3. The Breakdown Crane is a crane smaller than Rocky, but bigger than D10. He is used for the jobs that the others can't reach, due to his smallness. Everybody loves him, and chooses him for everything but gross jobs. He is Mike's crane of choice, and has repeatedly won Crane of the Year Award, much to the dissapointment of Cranky and Rocky. There is nothing he can't stand, because he's AWESOME!!!!
4. The Drivers and Firemen let the engines do whatever the hell they want, due to the fact that the trains can control themselves. They love it when the trains wreck, and laugh when Sir Topham Hatt yells at the trains for wrecking themselves. They still have never been found out. These people can't stand the thought of being caught in the act of wrecking a train, and all being arrested.