Things are different now in the US. The US was a desired target and a train would've been far easier than the (alleged, let's not get into that...) terrorist takeover of the airliners. Further, besides the nuts in CA, trains are not a primary public transportation method in the US.
Yes they are extensively used if you include subways, but on the scale of things knocking out a train car - rather than a station attack which is just a public gathering that has little to do with there being a train nearby, is just another populated area) - isn't as much collateral damage as occupying a middle eastern country for a few weeks.
Perhaps it would make news but logistically they'd hurt us more knock out a material supply line than a passenger trail.
While it's true that terrorism is about terror, it's not that simple. It's about terror to affect a change. Again we have to look at the significance of what the world trade centers were, it's not at all the same as just making people afraid to ride a train.
If you dismiss that there were wealthy companies in those buildings, reconsider exactly what went on in them. It's not like they randomly picked two buildings, nor picked them only because of their height... but if you want names, here you go -
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