If I were in your shoes I would first drive to your new location and see how your current scanner with a mobile antenna hears. I would also check some topo maps to get a rough idea of what the path looks like.
If your new location isn't reasonably clear of obstacles and the path isn't good back to your former location, it will be difficult to realize enough gain to hear the handheld radios that you are used to hearing now. You will probably still be able to hear the cab radios in the locos and the dispatcher but not the handies.
I'd then decide based on the path, whether I was going with an omni directional antenna or a directional antenna. Judging by the coverage that you want, I'd be tempted to go with a directional and high gain antenna pointed towards the original location 20 miles away.
Once that is installed you can try a preamp.
I have not had good luck with scanners made by GRE. We have a 25kw fm station across the river, with an external antenna the GRE scanners that I have tested on an external antenna become severly overloaded and unusable at 160 mHz. The GRE preamp that goes on top of handhelds had the same problem.
Some radio shack scanners are made by GRE. I tested a pro 94 and 95 I think they were. Both 1000 channel scanners. The newer one, and less expensive one, with more banks, and some more features was made by GRE and was totally overloaded with an external antenna and unusable at 160 mHz. The other one, the older 1000 channel scanner had no problem at all.
My old Uniden BC 200 also had no problem. A couple of other RadioShack scanners not made by GRE were also fine on the external antenna.
So, that is one of the pitfalls of an external antenna, it tends to highlight weaknesses in a receiver.
Your other choice may be to do what some others do, and that is to have a remote receiver that you listen to over the net. There is one in Hebron Maine that works very well. The guy set it up so he could listen to his scanner at work.
If you know someone with an internet feed who is in the area, a scanner and an older pc and net connection would do the trick.
I am considering putting a remote reciever at a friends house who is up high on a hill. Winamp.com has the software to set it up.
This solution has it's drawbacks too but if you have netsavvy friends who are in the area, they probably won't mind you using a bit of bandwidth to listen to your scanner remotely.
Go to winamp.com, do a search on Maine scanner radio and you should be able to listen to his feed. Works pretty well. When it's on.