The biggest hurdle is the size-vs-speed question:

• The bigger the wheel, the faster you can go.
• However, a 36" wheel is never going to be accepted on the Metra during rush hour... the smaller the wheel, the more likely I'll be able to take it on the Metra. A 20" wheel is optimal, but they're too slow.
• Shorter crank arms can also result in faster speeds. BUT they also mean you can't stop very fast.
  • Maybe you can compensate by having REALLY beefy legs?

• Internal gear hubs actually let you change gears. But ALL are >$1000 just for the transmission, so that's a non-starter. At that price, folding bikes are equally viable. (conclusion: chain-based transmissions are fundamentally cheaper than internal-hub transmissions)
• there's the "jack shaft" design alternative, but those are one-off only... crazy...