So far I've a couple of loose options;
Orlando - Sacramento flight
Consider the California State Railroad Museum as long as you are in town.
Pick off Donner Pass, highly weather dependent as if its like this year with nearly 800" of snow then I'm not going to get up there even at the end of April, but I'll pick off Gold Run, Colfax, Dutch Flat for certain. Down side to Donner is lack of traffic, 20-25/day.
I've never chased trains over Donner, just driven across on the way to Sacramento. But there will likely be plenty of snow still at that time of year. Also, I think you will find that there aren't a whole lot of mountain routes that see really heavy traffic these days, unless maybe you hit the Canadian Rockies? Or maybe the BNSF route east from Seattle? There's a reason they run the trains around the mountains when they can. You won't find a high traffic flow over the old DRGW in Colorado either.
Then option one is to drive down to Techachapi and pick off Bealville, Caliente etc for a couple or three days. Better traffic flows but not mountains hopefully Donner will have satisfied that itch. Follow that with two or three days down at Cajon, plenty of traffic here with 100/day.
I'm not especially impressed with Techachapi. It's apparently a mystical, mythical place, the product of amazing engineering. It looks to me like a curve in the track that just happens to keep going until it circles over itself 
. Do that inside a tunnel, twice, a la CP spiral tunnels, and I'll buy in. Anyway, it's cool enough, and everything is cool when trains are running, but I'd give it an afternoon or day at most and spend the rest of the time around Cajon. We spent too much time getting to Techachapi and ran out of time on Cajon, which is a mistake in my opinion. There are lots of trains over Cajon, and lots of good photo locations up and down the hill. There's also a pretty good railfanning guide somewhere out there on the internet. I don't remember the site at this time though. I may have a copy saved on my other computer. If so I'll forward it along.
After that not really sure, two options;
1a, Head East, pick off Needles, Kingman and Flagstaff and try to get back to Denver then flight back to Orlando.
There will be plenty of trains along this route, but again not much mountain railroading. But the scenery is spectacular in a different way. And that's my issue with this alternative. I know your focus is trains, but I think if you pass through this part of the country it would behoove you to set aside at least a few days to see the area attractions, even if they don't feature trains. To be in Flagstaff (a cool town in my opinion) and not make time to visit the Grand Canyon is a tremendous miscalculation. There are so many amazing and different things to see out west. I encourage you to take your family somewhere besides Orlando on one of your trips. Anyway, good route especially if traffic volume is your primary goal. Not so good if mountains are important.
1b, Head NE to Las Vegas, Salt Lake City, Laramie, Cheyenne, Denver.
Decent volume this way, even more once you reach Salt Lake City. Nevada is a snooze though. But I think Utah and the Wasatch Mountains east of Salt Lake is a great environment for trains and scenery. Weber Canyon has some cool shots and tracks seem pretty accessible. The UP main across Wyoming is generally accessible from Interstate 80 without too much sidetracking. Lots of trains. Not much in the way of striking mountains, like the Colorado Rockies. Lots of people think Wyoming is kind of boring. I think it's pretty interesting in its own way.
The alternative is to follow the old DRGW out of Salt Lake City, across Utah, and through the Colorado Rockies into Denver. No where near the traffic volume as the UP main in Wyoming, especially with coal traffic off sharply. But real mountains, and some places where proximity to the tracks is very immediate. If you go this route, I can definitely lay out some of the back roads to follow to stay close to the tracks.
Option two, Donner for two or three days, then drive up to Portland and pick off the Columbia river gorge for two or three days, again 40/ day traffic flow and then head down to Boise, Salt Lake City, Laramie, Cheyenne and Denver.
I probably like this route a little better than the Arizona route. I didn't get a chance to follow the Columbia River all the way west, as we headed north up to I-90 then over to Seattle instead. But the UP through Oregon parallels the highway in many places. Decent mountains, although not quite Rockies magnitude. And again, once you get down to Salt Lake City and east, there is a lot of traffic.
On top of all that, whilst in FL hope to pick off Mulberry, Dade City, Wildwood and Ocala but that's one day and then a two day visit to the Folkston Funnel for a camp out there.
Couple of things I still need to research, drop in flights, eg just drop in a the desk and fly anywhere internally.
I would personally try to have reservations in place for flights. I'm sure there are instances where you could walk and buy a ticket, and maybe even last second deals, but I don't know how reliable that is these days, and I'd hate to think what you would pay for those tickets. Most flights I've been on in the past several years have been solidly filled, with any empty seats taken by standby passengers. I can't think of the last time I've flown where there were more than one or two empty seats on the flight. This is on airlines like Southwest and Frontier, which are considered discount airlines, although not super cheap discount a la carte lines like Spirit or Allegiant. Delta and United are usually fuller service and fuller price flights. Motel drop ins, it pays to book in advance but being as I've no idea where I'm actually going that's a bit difficult.
Unless something in particular is going on, like spring break from school in late March/early April, you should be able to dig something up. Especially if you are ok with a night in the car here or there. Maybe check availability once you settle on a route. No issues with spending the odd night in the rental vehicle (is that legal) lineside so that I get the pre dawn and dusk shots, Cajon is a definite, Bealville another possibility.
Anyway, back on topic, I'd love to see your work flows on those items as I've got quite a few bits I'm going to need for up coming US projects
I need to get the software loaded and figure out how to do some videos. It will be the fastest way to explain things.
MD
.....etc