IMHO Charging this fee would put merchants at a competetive disadvantage and not be profitable.
Many merchants seem to disagree.
I don't purchase motor fuels, since 90% of my local trips are walking (and the vast majority of the rest of my local trips are by public transit), but I note that lots of motor fuel vending outlets charge different prices for cash vs. credit. If it caused them to be unprofitable, they wouldn't do it.
Plus, many places used to have unofficial policies in place that either had a minimum purchase amount, or a surcharge (or both) on credit card purchases, even though they technically weren't supposed to do that according to their credit card agreements.
Many small businesses run on really thin margins, and don't have a lot of overhead or other costs they can reasonably cut. So when they're faced with fees for accepting payments (fees which can run, in some cases, 5-6% of the sale), that actually puts them at a disadvantage and can turn a profitable sale into an unprofitable one.
The larger the vendor, the less of an issue this becomes for two reasons. First, high-volume sales outlets get lower fee rates than low-volume ones do. Second, when it comes to cash handling, that in itself can become expensive if you have to hire employees and trust them with your cash, plus have a place to collect and store large amounts of cash, plus have some kind of way to transport that cash to the bank, you wind up with very high costs (or potential costs, in the case of theft) for cash handling. In that case, credit cards become considerably more cost-effective.
Checks are an entirely different story. These days, many places no longer accept checks (and this includes Amtrak), and banks are now charging higher and higher fees for processing checks, in part simply because they have to maintain check-processing infrastructure for a dwindling business (on a side note, it wouldn't surprise me to see the personal check simply disappear in 5 years; or at least require a high-fee account to be able to have them).
So, for now, at low-volume, low-overhead businesses, cash is cheaper to handle, and at high-volume, high-overhead businesses, credit cards are cheaper to handle.
Amtrak clearly falls into the latter category. In fact, I believe the Lake Shore Limited PRIIA report includes a proposal to go to a cashless diner. Reducing or eliminating cash handling on trains would greatly streamline the operation.