I'm sure this will provide a useful resource to many here, so I'd like to point this out. At any rate, it's neat. DeOldify is a program that automatically colorizes black and white photos and videos by employing machine learning algorithms. It's free and web-based, but since it's hosted by Google Colaboratory you'll need a Google account (such as your Gmail account) to access it. Simply follow the directions on the site and provide a URL to an image online, and a few seconds later you'll have a colorized photo. I've been playing with it for a little bit, seems to do a decent job colorizing old RR photos available online (see attached).
Here is the link to their homepage: https://github.com/jantic/DeOldify#about-deoldify
And here is the link to page where you "check out" and use the software: https://colab.research.google....ColorizerColab.ipynb
As an example, I've attached several comparisons of the original black and white and then the color photo that I've colorized using it. They're of PRR's Brady Tower, near East Brady, PA, along the Conemaugh Division, as available from the RR Museum of PA's online archives. Pretty cool!