Hi Dan:
The 55-250 lens that you mentioned in your response is the one I would recommend. That should do the job very well and it is an economical solution that has good optics.
I would not recommend an extension tube. They are normally used for extreme closeup work, such as macro photography. What is normally used to extend the focal length of a lens is an extender, but an extender will not work on either of the lenses I suggested and would be much more expensive in any case. It's hard for me to assess how big the train will be in your image, but the magnification factor of the 55-250 is obviously going to be 5x over that of your 18-55, FWIW. I would recommend just getting the 55-250 and see how that works for you.
Reviews from reliable and respected sources are valid, and I do offer only valid ones. Ernie has a thing about anyone's opinion but his own and he is entitled to that. I research carefully before purchasing gear and in 40 years it has worked for me - in the end we must each make our own decision.
Good luck with your grandkids.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
