Sure, the mileage bonus and the guarantee are all well and good, and I'm generally an advocate of DL, but DL stirs up a kittle of fish with the plane/train comparison.

Admittedly, if you're going from an LGA hotel to Crystal City, VA, the Shuttle is generally best. But if you're going between Manhattan and the District of Columbia, Amtrak is very competitive in terms of TOTAL time, not to mention legroom, restroom availability, and price (especially considering cab fare to/from LGA).

Further, Amtrak is not often delayed by weather. I wonder how many people stuck at LGA right now (while I'm sitting in CVG with a major weather delay due to storms near LGA) are wishing that they'd stayed in Manhattan and taken a train?

(I also wonder about gallons of fuel [and consequent pollution and trade imbalance] per passenger on Amtrak vs. the Shuttle, but that's another subject.)