Going with the previous two entries here are some reasons why heavy rail options really take the lead when vast volumes of people are trying to move through a corridor of travel.  The North East Corridor comes to mind as a prime example.  Even the most ardent supporters of auto, pro-bus, or other forms of transport can’t argue against the efficiency of rail when this volume of passengers are concerned.  With Amtrak, New Jersey Transit, and numerous other transit agencies running trains in and out of the major cities one can expect to see trains every 2-5 minutes, or even every minute during heavy travel times, in this corridor.

Legitimate Reason #1:  Heavy rail passenger volume can't be touched by any other mode of transport.  In addition the speed it attains is very high for such volume, often higher than automotive transport which usually leads other modes.  But the number one reason heavy rail leads the pack is the ability to achieve volumes in excess of 50,000 and some volumes as high as 70,000 riders per hour.

Legitimate Reason #2:  Heavy rail is generally the cheapest thing going once total costs are considered.  This is absolutely true for high volumes, but even for low volumes rail often offers a low price point per rider.  When extremely low volumes achieved, often that does not validate commuter unless their is existing track that is already usable without upgrades.

The WES is an example of economically unreasonable commuter rail.  TriMet will need to achieve over a 400% increase if the WES is to achieve the same price parity as bus service in the corridor running the same passenger count.  Approximately 800% within the next 2-3 years to achieve the same price parity as light rail service per passenger.  That is based solely on operations.

On the other side of the spectrum, the Sounder Commuter Rail Operations in Seattle is approximately 30-35% more economically efficient than bus service.  In other words, it costs 30-35% less to carry a single passenger via the Sounder versus their similar bus service.

Another prime example of passenger rail service that is successful, yet has low volumes is the Cascades rail service between Portland and Seattle.  Above rail operations are almost break even (one more frequency is needed).  The service operates at approximately 25-30% less than comparable bus service operations.

Legitimate Reason #3:  This one can be argued, but with a great deal of absurdity.  Passenger rail is almost always the most comfortable, smooth riding, fastest, efficient mode of transport to carry passengers – low or high volume.  Now mind you, the privately operated buses on the east coast and some other cities, that offer real space and amenities close to passenger rail often do provide a similar level of comfort, but those services are rare.  Almost all passenger trains in the US though offer 2×2 seating, sometimes 2×1 seating, wide aisles, the ability to walk about while in transit, and other amenities of this grade.

The other reasons include almost all of the ones I’ve outlined in the previous entries for streetcars and light rail.