Amtrakking,
Just to be clear, you have an area 16' X 4' and another section 9' X 4', right?
I think the first thing that needs to be mentioned is Radius. Your maximum radius is going to pretty well dictate what you can and cannot (or may not) be able to run. HO virtually dictates a minimum radius of 22" in order to run any modern equipment and/or any 6 axle trucks. With the dimensions of your possible bench work, you could have 22" radius curves at either end of the bench work. Therefore, you could run almost any HO scale train, but only almost, possibly not all. Your Amtrak, and most other trains passenger and freight) would work with 22" radius curves.
"A reasonable layout" ...
What constitutes a reasonable layout is really up to the individual and must take into account what the individual wants/expects from their layout.
When I built my first HO layout I thought I had a "reasonable layout" and aesthetically it may well have been BUT it didn't provide me with the things that I really wanted - modern engines, longer trains and more opportunities for scenery. As such, I ripped the HO up and am putting an N Scale in place, a scale that will give me the things I really want.
What options do you have with HO? I would think that you could have virtually everything you wanted but in very reduced quantities and sizes. An Amtrak train (engine + 4 cars) is 52" long in HO Scale. The same Amtrak train in N Scale is 31" long. Freight trains are realistically longer but they would be restricted to probably no more than perhaps 8 cars to maintain some sort of proportion between train length and track length where as in N Scale you could probably run 15 - 20 cars and still have a sense of realism.
Scenery wise, you would be able to have some very nice features, a town, mountains, tunnel/s, bridges etc but it would be fairly close to one another due to scale. You would get approximately twice to three times the amount of scenery in the same area with N Scale; therefore, allowing you more options and to be able to incorporate more things such as industries.
At the end of the day though, what you consider to be a reasonable layout is in the eye of the beholder, the individual. Which ever way you look at it, you really do need to think about what you want and what your expectations are. Trust me, there is nothing worse than building something because it is convenient while for going what you really want. Doing it that way may please you in the short term BUT you will always want what it is you REALLY want from your layout.
Hope this helps and believe me, I'm not trying to pressure you into one scale or the other. I'm just taking into account the things you mentioned you wanted, as that is important and necessary and a must. At the end of the day the decision is yours but hope the comparisons I mentioned might help you to choose.
Good luck and look forward to seeing whatever you choose.