Love your Casey junior train pin set!
You will have a lot of fun searching and seeing pin sets of trains. I will have to look through my collection and see if I have any to share, but if you are looking for a few to get ideas for, here are some sets that have caught my eye over the years :
Mickey & Minnie's Runaway Railway Connected Train Collection Disney Trading Pin Series | Pin & Pop
^ My mom has some of this set of Mickey and Minnie’s runaway railway and it’s a cute depiction of all the characters and connects together
DL - Train - Mystery - Set
If this link works, this is a pin set that came back a few years ago and I adored how it depicted a lot of the Disneyland resort rides all together - also can be connected with one another
BoxLunch - Inside Out 2 Emotions Train Set - Mystery
And this one is of the inside out emotions all in a train. I don’t know if it’s a movie your son enjoys, but the colors are so bright and it’s a really cute and pops out to me
Just a few to share if it helps brainstorm collecting ideas. hope it is of help or enjoyment! I’ll survey my family’s books next time we enjoy a pin day and I’ll try to snap some pics and post them here