IMO, the most significant thing is to have as close to zero debt as possible. The less you owe every month, the less you can make that month and still succeed.
Is your wife on board with this? That's actually more important than number one.
You can charge higher rates for working on expensive stuff than you can working on cheap stuff.
If you are sticking with trains, that means being a top notch repairer of the newest command controlled trains. It also might mean that you can work on high end Standard gauge trains, where the skills are quite different.
Are you generally handy and love to fix things? Maybe you could add smart phone repair. Cases, screens and tools are readily available online from China. You would need a small stock of the most popular parts. "How-to" videos are on YouTube for almost everything that is current/popular/trendy.
