Getting tickets for MARC is a fairly confusing process. There are various ways to get a ticket, all of which have their own idiosyncracies about them:
(Update: MARC fixed things in 2018. See the Postpostscript.)
At stations that have them, you can buy tickets from Amtrak ticket machines. This includes not only the stations that are also Amtrak stations (Union Station, New Carrollton, Penn Station, Rockville, etc.) but also some other stations that are not served by Amtrak (Camden Yards, Savage, etc.) These machines only accept credit/debit cards, not cash. Many stations do not have these machines.
If you're boarding at a station that doesn't have an Amtrak machine, you can buy a ticket on board. Conductors on board only accept cash, not credit/debit cards. Additionally, there is an extra charge for paying cash on board if you boarded at a station with an Amtrak machine.
You can buy tickets in person at local commuter stores, almost all of which are located in Virginia, where MARC offers no service. (2015 update: there will be a commuter store at the new Sarbanes Transit Center in Silver Spring, which opens next week. Huzzah!)
You can buy tickets in person at most Amtrak stations where MARC stops, if you can get there by some other means.
You can buy tickets in person at the MTA office in Baltimore, if you can get there (it's in the William Donald Schaefer building.)
You can buy tickets by mail, if you are buying a weekly/monthly pass.
I live in Greenbelt, which has a MARC station with no machines and no ticket office. If I want to ride the train, and have not had the forethought to buy a ticket ahead of time, I must have cash on hand to buy a ticket on board. Otherwise, I need to find the nearest ticket office, which is in New Carrollton and a 45-minute bus ride away, or the second nearest, which is at Union Station and a 30-minute Metro ride away. Since I rarely carry cash any more, this becomes sub-optimal.
So I've been thinking recently about how MARC could improve this situation, short of putting Amtrak machines at all stops, which they seemingly do not want to do for whatever reason (or else they certainly would've done it by now.) Two approaches come to mind, both demonstrated by other commuter rail systems in the country.
The first approach would be to accept SmarTrip/CharmCard, as MTA does for local transit services. This is what Caltrain and Sounder do using the Clipper and Orca cards (the Bay Area and Puget Sound equivalents, respectively, of SmarTrip/CharmCard.) You tap a card reader when you board a train, and then tap again when you get off. The system calculates the appropriate fare and charges it to your card.
Advantages: There are lots of these cards in circulation; many people riding MARC already have one, and thus it would be one less thing to ask commuters to manage. MTA is set up to load commuter benefit funds onto these cards. SmarTrip/CharmCards are fairly easy to acquire these days, with machines in Metro stations, sales at Giant & CVS, etc.
Disadvantages: You have to remember to tap off. Caltrain handles this by deducting the maximum fare when you tap on, and then refunding some of it (assuming you're not actually paying the maximum) when you tap off. MTA would have to do something similar. There would be some cost to MTA to install card readers at all the stations, as well as to give conductors the same card readers that light rail ticket inspectors currently carry. Unless MTA installs something like the light rail ticket machines at all stations, you'll still need to load funds onto your card somewhere else, and if MTA can do that, why can't they put Amtrak machines everywhere?
The second approach would be to have a smartphone app for purchasing electronic tickets. This is what the T does for commuter rail around Boston, and NJ Transit does for commuter rail in New Jersey. Amtrak also offers this across the system. Using these apps, you buy a ticket ahead of time, store it on the phone, and show it to a conductor when they do their ticket collection.
Advantages: Doesn't require MTA to install any new equipment at stations. Lots of commuters have smartphones.
Disadvantages: Lots of commuters don't have smartphones. Conductors need to be trained to recognize valid e-tickets and, depending on how strict you want to be, need to be equipped with handheld code readers to verify tickets (Amtrak does this; the T, as far as I can tell, usually does not.) Unless your e-ticket purchases are backed by an online account (they are for Amtrak and NJT, they aren't for the T) you are screwed if you lose your phone. Cash purchases wouldn't be possible. Loading benefit funds might be hard.
So neither of these approaches solves all the problems, but either one could work at least for the Greenbelt scenario. Greenbelt is a Metro station, so SmarTrip cards and reloads are readily available, and so the SmarTrip-based system is viable. I think I'd prefer that over the smartphone approach-having one card for all transit-related needs, and not needing to carry my phone (although I always do, anyway) seems like the best solution to me.
(Tangential to all of this is that SmarTrip/CharmCard's days are numbered; eventually it'll be replaced with a system based on contactless credit/debit cards and NFC, as is happening in Chicago and Philly. That's a subject for another post, which I'll probably write next year at this rate.)
I wrote this article two years ago (as of the date I'm writing this sentence.) Nothing has changed for MARC in this department. (However, MARC does have new Bombardier bi-level rail cars, similar to those used on NJ Transit, so that's nice.)
I neglected to consider a third approach, which is basically a combination of the first two: a smartphone app linked to a SmarTrip card. This is what Metra is going to do to join Ventra and thus make Ventra usable across Chicagoland transit. Metra had a page (which seems to be offline now) explaining why they didn't go with my first approach above - basically, Metra has a lot of stations, and the cost to put tap-on/tap-off readers at each station would be prohibitive. In addition to allowing Metra tickets to be bought with Ventra, the app would also provide Ventra account management services and, eventually, permit you to use your NFC-equipped phone to pay CTA and Pace fares from your Ventra transit balance (as opposed to your credit/debit card balance, which you can do today - basically, it'd be like using your Ventra card without actually using the card.)
MARC has approximately 1/6 as many stations as Metra, so maybe the first approach wouldn't be as cost-prohibitive for MARC. However, the current governor isn't a fan of spending money on transit, so perhaps that's irrelevant. Nonetheless, I like this idea of combining an existing smartcard infrastructure with an app. Perhaps WMATA and MTA can get together and come up with a plan, the humble narrator said, ignoring for the moment that MTA has to beg for money and WMATA has more problems than that Jay-Z song.
It's now 2018, and MTA has finally made some changes to the MARC ticket process. The first is that there are now MARC-specific ticket machines, which have been rolled out at more (but still not all) stations. All of these machines accept credit and debit cards via magstripe, EMV (chip) or NFC. Some machines also accept cash. This is a welcome step and I hope MTA gets around to putting these machines at all stations, since Greenbelt still doesn't have one (but College Park, which is nearby and has more trains stopping there, does, so apparently the most-used stations are getting the machines first.)
The second is a new mobile app called CharmPass which allows you to buy tickets for not only MARC, but the Baltimore local services (bus, light rail, Metro subway) and commuter buses as well. You can buy either single trips or all the different types (as far as I know) of weekly and monthly passes. Apple Pay is accepted in the iOS app (I presume the Android app supports Google Pay, but I didn't test that one) along with the traditional method of entering your card number. It's made by Moovel, a division of Daimler that has made apps for a bunch of other transit agencies, and the app seems to work well enough.
CharmPass joins CharmCard as a means of buying MTA fares, but doesn't replace it. Since a CharmCard is basically a WMATA SmarTrip card with different artwork, running on a system operated by WMATA, this gives MTA a system that doesn't depend on the increasingly dysfunctional WMATA; should WMATA downsize itself into the ground as it appears intent on doing, MTA could continue using CharmPass and migrate its CharmCard customers over. Intentional or not, it's a shrewd move. (CharmPass is presumably dependent on Moovel technology, so it depends on Moovel continuing to be a going concern, but that seems to be a good bet at this time.)
The only slightly silly thing about CharmPass is using it on the Metro Subway. Because the turnstiles can't recognize a CharmPass ticket, you have to show the pass to the station manager, who will print a paper ticket for you to insert in the turnstile. That seems wasteful, but I hope it's a short-term solution; if MTA is serious about CharmPass, they should look into new turnstiles (and perhaps bus fareboxes) that can scan the barcode from the app.