It could be that AQ wants to divert everyone's attention to the rail system then while everyone's focused subways, etc., they go for their big-ticket targets (i.e., malls, landmarks, maybe even N-plants).
They have been very patterned over the years. Evaluating the pattern is one of the first, basic methods of evaluating an opponent.
First the car bombings throughout the ME and carried over into Iraq. First WTC attack, Beriut, Saudi Arabia, African embassies to name a few.
The 911 attacks were outside of the box for them, except for the coordination.
Now a second, non-ME metropolitan attack, focusing upon mass transit. Seems to be a pattern developing. I do not discount car bomb attacks against malls, they would be a high priority IMHO and within their MO and capability. However, their MO doesn't support attacks against landmarks or Nuke plants.
Because of personnel and resource requirements, terrorists will generally stick with a tried and proven method of operation. Those with the least potential for discovery are favored. Training and preparation are more simple for a suicide bombing than for a more complex attack upon a nuke facility.