I've used many cameras to railfan and to shoot in general. I have a Nikon D750 which is a great camera, great autofocus, and low noise under ISO 6400-12800 in some conditions. I use manual mode so im not sure about the auto mode functions, it should be ok.
I have also used a Sony A6500 A7rII, Canon 70D and Nikon D4. The Sonys are nice and small, but the battery life is short, not sure im ready to make the move to mirror less. I like my D750, just the right size and resolution for trains and shoots with m friends.