Well, purchasing another set is an option. In the classified sub forum, members have trains listed for sale. Many of the posts may be old, but it is quite possible their trains are still for sale.
Yes, I do have trains listed but they probably aren't what you want to expand your empire.
If there are any train shows in your area, check them out. You never know what you will find.
Local hobby shops & craigs list are options for finding trains.
What ever you acquire, make sure it is compatable with what you already have.

Happy Rails!
Dave

A CHILD AT HEART, STILL PLAYS WITH TRAINS!