Hi there!
Happy to see that you've worked through those lessons! Yes, Freight Train is a good song to try out after this! On our site, I'd suggest you try either of these four lessons next:
"I'll See You In My Dreams* lesson
"Salty Dog Rag" lesson
"The Wellerman" lesson
"Folsom Prison Blues" lesson

Have fun! And let us know if you have any other questions!
Kind regards/Emil