Schumer, citing U.S. intelligence analysts, said attacks were also considered on Christmas and New Year's Day and following the president's State of the Union address.
He called on the U.S. Department of Homeland Security to expand the Secure Flight monitoring program, which cross-checks air travelers with the terror watch list in an attempt to prevent anyone on the "no-fly list" from boarding, for use on Amtrak.
Such a procedure would create an Amtrak "no-ride list" to keep suspected terrorists off the U.S. rail system, he said.
This is one of those situations where reasoning by analogy can lead you seriously astray. Airplanes are an unusual case not so much because they are uniquely vulnerable, but because they are uniquely secure. It's true that planes are relatively fragile in that a small amount of explosive can kill a lot of people, and that even small accidends tend to kill everyone. [This is only a partially unique property, though, so a full account of why planes are such an attractive target surely needs to involve some social and psychological factors.] However, they are also ordinarily well protected, so it's hard to get access to them to do damage. At least in theory planes are kept in secure conditions on the ground so it's hard to place a bomb, and obviously once they're in the air it takes something like a surface-to-air missile (or least good luck with a gun) to cause catastrophic damage. This means that if you want to attack a plane, it's very convenient to actually be on it, so it's at least arguably useful to keep suspected terrorists off the plane.
But this doesn't apply to trains, which are (a) not particularly well secured and (b) easily accessible when they are in transit, which means you need to secure hundreds of miles of track. This means that if you want to attack a train, you don't need to be on it, you just need to get access to the track and damage it at the right time. (See here for a list of train accidents). It's like these guys have never seen Bridge on the River Kwai.
Even if that weren't true, it's important to remember that while planes are already a limited-access type thing, trains often are not. Amtrak may do some kind of passenger identification there are lots of commuter trains (e.g., Caltrain) where that not only aren't passengers identified, you can get on the train without a ticket. Instead, the conductors just come by periodically and audit. Converting to a system where you actually checked ID for each passenger before they got on (and remember that something like 50-100 people might get on in 2 minutes on an open platform) seems like it would be prohibitively expensive. These trains don't carry as many people as some Amtrak trains, but they certainly have enough passengers that if you could kill a significant fraction of them it would be bad. And this doesn't even get into the question of subways. In general, train security is set at a level designed to deter fare evasion, not to protect the train itself.
Even if you think that airplane security is set at an appropriate level (which IMHO it probably isn't) this seems like a security measure which comes at a huge amount of cost and very little benefit.