Hello and welcome!
Sounds like you’ve got a great pike and a great way to spend time with your son.
You didn’t mention if you’re running DC or DCC.
Rather than having one huge locomotive you might consider stringing several smaller locomotives together. This is known as consisting or MU (Multiple Unit).
A single 8-axle DDa40X is rated at 6,600 hp, a GP-40 (4-axle) is rated at 3,000 hp and a GP-30 (4-axle) at 2,250 hp. So two GP-40’s would come close to the hp rating of a Dda40X and would be able to negotiate the 18” curves.
You can go to THE DIESEL SHOP website (
http://www.thedieselshop.us/index.html) to look up the specs of different locomotives and power ratings.
To MU you’ll need locomotives that run at the same speed. Generally locomotives of the same type and manufacturer run pretty closely together, but not always.
If you’re running DC you’ll need to find out which of your locomotives run at similar speeds. You can do this by timing each locomotive around a loop. If you are running DCC you might be able to speed match these electronically.
Google Speed Matching.
I have 15” radius curves on my pike and with a consist of three GP-40’s I’ve had no problems.
Hope this helps.