It is my understanding that most radio services will have to convert to a narrower bandwidth at some future date. I have heard that the year 2012 has been designated for some services like Public Safety communications. To do this, many agencies are going to digital, which is very narrow banded. Most current analog transmissions do not meet these new FCC mandated, narrow band standards. I am sure the railroads will have to adjust, too, at some point. The reason for all this is to conserve radio spectrum. The cell phone industry, in particular, is gobbling up spectrum like there is no tomorrow. TV transmissions are supposed to go ALL digital by the end of 2006, but many doubt that will happen by then. Also, the TV spectrum above Channel 60 is being withdrawn and redistributed to mobile radio, cell phones, etc. This means there will soon be NO TV stations above Channel 60.
I think digital scanners will be getting cheaper in the near future and all of them do receive analog signals. So, I would probably get a digital scanner to cover more future options.
The thing to worry about, in my opinion, is encrypted digital which many police agencies (and others) are going to. There are NO scanners that will receive such transmissions (and probably never will) due to new laws that prohibit reception of encrypted signals by unauthorized parties.
Another thing about digital which can get pretty "deep" is the fact that there are many different formats of digital transmission and no scanner will currently work on all of them. Many agencies are adopting what is known as APCO-25 standards for digital transmissions so different services can easily communicate with one another. As more and more agencies are going digital, they are finding all sorts of incompatibilty problems for inter-agency communications. If they all adopt APCO-25 standards, they have a better chance of being able to communicate with each other.
This may seem a bit complicated as far as railroads are currently concerned. I have noticed, however, that CSX and NS have all applied for trunked radio systems which are being experimented with in some areas... So, this means you better get a trunking scanner, too. And, again, there are several trunking systems out there (Motorola, EDACS, LTR, etc.). Most scanners will usually only work with one or two trunking systems.
Ahhh.... for the good old days.... when we just plugged in crystals....