It all depends on the capacity of the train and how frequently it stops.  Starting and stopping are the real killers since you need extra spacing (or very long &quot;onramps&quot;) to accelerate and decelerate a train.<p>However, one concept is a train that never needs to start and stop.  Passengers move onto a separate car that separates from the main train and can decelerate into the station, and then it reaccelerates and attaches to the next train.  Conceptually, this allows you to run much larger trains since you don&#x27;t need to decelerate it every 10 minutes for a stop.<p><a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=DIeRrU4_M3Q" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=DIeRrU4_M3Q</a><p>This is the original concept.  Let&#x27;s just say I have some doubts about the feasibility of engineering that design in real life, but the concept of detaching cars seems sound to me.  Conceptually I do like the idea of the detachable cars being a different type of unit with a higher top speed and better acceleration&#x2F;deceleration units to assist in the deceleration and overtaking steps.<p>A simpler implementation would be to have self-propelled carriages that combine motors and passenger space, and have the car accelerate and decelerate on a siding.  The main train would have to run at a slower speed than the detached car could achieve in order for it to &quot;catch up&quot; but could then accelerate back to top speed once reattached.  This would lead to an &quot;oscillating&quot; pattern of distance between multiple trains on the line.<p>Also, at the end of the day a runway is still a &quot;one-dimensional&quot; unit of capacity in the same way that a rail line is.  You can have multiple multiple runways or rail lines (which I guess is &quot;two-dimensional&quot; in your definition here) but you certainly can&#x27;t have multiple decks of runways or land multiple airplanes on top of each other.  It&#x27;s actually perfectly feasible to have rail lines running at multiple depths, many subway stations do it.