Like the Northeast Corridor, the West Coast Main Line, running northwest from London, would classify as regional high-speed rail by the Federal Railroad Administration, if such categories were relevant in Britain. Despite the recent upgrade, the line is approaching capacity and the British government is planning to invest £30 billion in "HS2", a new high-speed rail line that would parallel the exiting line. (They've also changed the name of the Eurostar link from Paris to London to "HS1", implying there's more high-speed rail to come.)

The British government is making these investments for a number of reasons. They will promote economic integration between Britain's major cities and regeneration in its under performing ones. They will widen the labor pool of major English cities like London, Manchester and Birmingham. They will provide better access to one of the world's busiest airports -- provided the government can come to agreement about the right approach for connecting to Heathrow. All these are valid. But the main reason the line will succeed is because every seat on my Monday morning train to Birmingham and afternoon train back to London was full, and there were three more just like it in the next hour.

High-speed rail works when the lines are built parallel to existing lines, once those are at capacity. Expanding capacity of transportation networks is vital in advanced economies in which businesses rely on face-to-face communication when congestion threatens to stifle growth. And if you need new capacity in a high volume corridor, why not build the state of the art?

Paris to Lyon is a classic example of a successful high-speed rail corridor. It essentially wiped out air travel between the two cities. The new high-speed line was built parallel to the original link, after it was at capacity. Tokyo to Osaka, the highest volume high-speed rail corridor in the world (and the first-built), operating now for nearly half a century, was built because the existing corridor was at capacity. In fact it is so successful that the government is now planning an additional line alongside it to relieve it congestion once again. If it worked the first time; why not the second?

When a new high-speed line is built after the original line is at capacity, everyone benefits: the cities served by the new high-speed connection, as well as those on the original link now bypassed by the high-speed service. Under Britain's scheme you will soon be able to get from London to Birmingham in 45 minutes instead of 80 minutes. This will be good for travelers going from London to Birmingham; but it will also be good for people going to Coventry, a stop between the two cities on the existing West Coast Main Line bypassed by the new link. It means new capacity will be open for more local and regional service between Coventry and Birmingham and London. And the new service will be especially good for Berkswell, a city not on either the existing intercity service or the new high speed link, but a stop on the regional rail link from Birmingham. Until the new line exists, the government has prioritized high-speed travel on the existing West Coast Main Line, which has meant that Berkswell, like many smaller stops, has seen its service cut from 30 minutes to 42 minutes to accommodate the additional trips between Birmingham and London.

HS2 will relieve the West Coast Main Line of its goal of providing high-speed service between Birmingham and London, by providing that service itself. This will allow the West Coast Main Line to cater to growth in places like Milton Keynes or Northampton. Never heard of them? How about Princeton or Stamford? By only concentrating on top speed goals for the Northeast Corridor, we completely ignore 90 percent of the users of the corridor. But remember how all the seats were taken on my Virgin Train to Birmingham?

Well, they were full on my last trip to DC on the Acela too. Much of the Northeast corridor is at or nearing capacity. Building the infrastructure necessary to get from New York to DC or Boston in under two hours would be a boon to the regular business traveler on the new service, and they will gladly pay a premium for the time savings. But it would also benefit every user of the corridor whether the new service stops at their station or not. And if we are going to go out and buy ourselves new infrastructure, why not make it state of the art, and maximize those other benefits too, like agglomeration and regeneration right here in the Northeast.