Thanks for all the suggestions, and thanks Greg for the kind invitations - regrettably, our schedule meant we had limited time in San Diego on this trip but we liked what we saw and will be heading back next year for a longer stay.
I wasn't particularly impressed with the model railroad museum in Balboa park - lots of exhibits under construction or not operating, and we saw many families with young kids asking "why isn't it working?" Seems a missed opportunity to encourage youngsters with an interest in trains, and I heard at least one family grumbling about poor value for money. The one exception was the Lionel 3 railers group. These guys had a highly entertaining layout in full swing with many trains operating, as well as model cars, buses, aircraft and other line side attractions. They were also willing to chat with everyone who wanted to ask questions, unlike a particularly surly gent who was operating the only other layout that was working.
On the plus side, because we spent much less time in the model railroad museum it meant we had time to visit other Balboa Park museums - National History, Aeronautical and Car/Motorcycle - as well as take part in a Japanese Tea ceremony.
When we got to Salt Lake we took a ride on the Heber Valley Railroad, but apart from the "Wild West Show" outside the station - several guys very enthusiastically acting out a sheriff vs robbers scenario - the ride was quite dull. The fact that they don't seem to run steam locomotives any more (for environmental reasons we were told) was disappointing and meant we were pulled along by a nondescript diesel of some sort. Not one of our better preserved railroad experiences.
We looked into the Amtrak option for getting to Salt Lake from San Diego, but $600+ for the 2 of us (and that was just for a seat - sleeping berth would have pushed the cost to over $1000), plus a 34 hour journey with 3 am arrival time is just ridiculous. Our rental car for the whole of our 10-day vacation cost the same (and this included a surcharge for picking up in CA and dropping off in UT) and we did the journey in under 2 days with an overnight stop to see some relatives in Arizona. I really don't get Amtrak pricing policies.
Overall, we had a great vacation, just a pity the railways side of things was disappointing.