I have some experience with this issue. Capitol Limited, you don't need any
conversion for your beetle. My wife and I have been burnin' beans in our TDI for a year now. You just need a source for biodiesel, which can be a challenge, but it's slowly growing nationwide. In Colorado, I go to a fleet distributor and buy 30 gal of B100 in jerrycans. We blend it, very approximately, using about 30% bio and 70% dino diesel. That's enough bio to reduce the smoking, greatly improve the smell and give you bragging rights with your green friends!
Amtrak may be using biodiesel sooner than you think. One state, Minnesota, I believe, requires 2% bio in all diesel sold there. The engineer would never have reason to know. It works just the same, just doesn't stink when it spills on your hands and shoes. Now that's an advantage!
I wish the eastboud Zephyr had been burning bio last night. We got stuck for about 15 minutes in the Tunnel District of the Moffat Road, as the crew examined a boulder that came down near the tracks. Our Sightseer Lounge was stopped inside a tunnel, and the sulfurous stench of dino diesel was getting in. One woman traveling with us, a victim of acute chemical sensitivities, was breathing through a towel. That's as unpleasant a moment as I've ever experienced on a train.
If we'd been running B100, it would have smelled different. Some of us would have gone to the diner and asked when Amtrak started using deep-fat fryers! Soon, however, we crept past the boulder (with 4 inches of clearance) towards fresh air, and Denver.