Hi Tom, do you mean you want a telephoto lens? 17-55 is a zoom, ie it has variable focal length, whereas the 500/4 is a telephoto, in that it has a magnifying effect, but it’s not a zoom.
I don’t do train photography but I do a lot of dog racing photography and I imagine some of the issues are the same - the subject moving rapidly towards you being one. From that point of view a zoom is far handier, it being possible to zoom in whilst the subject is at a distance but zoom out to maintain a suitable image size as it gets closer.
Hi thanks for info probably the nikon 70 to 200 2.8 when I can afford it have the 17 to 55. Thanks
Thanks for all the info. Tom