The MBTA needs to explore a simple, low-cost (perhaps free) solution to one of the Green Line’s  most common and most noticeable problems: trains bunching together.  Dynamic looping, as I’ll call it, lets inbound trains get reassigned easily and transparently to different outbound lines.

Any regular rider knows that trains tend to come in pairs, and can sometimes emerge from the subway in groups of four or five at a time.  After 30 minutes without any service, a stream of trains will roll by together — the first few packed to capacity, and those behind nearly empty. Although this is a maddening failure to provide transportation, its causes are easy to understand.  Traffic lights turning red, inconsiderate or inattentive drivers stopping on the tracks, pedestrians running in front of trains, and other routine parts of city life all impose delays.

Every second that a train gets behind schedule allows more passengers to accumulate at upcoming stations.  A larger crowd of passengers takes longer to board, so the train is even more delayed at the next station, and the problem cascades.  Meanwhile, as the following train catches up, even fewer people accumulate at each stop so the later train can quickly find itself tailgating its predecessor with hardly any passengers aboard.

Many expensive capital projects could improve services.  With limitless funds, burying the B Line up to Packard’s Corner would be a smart move.  More realistically, the T could let trains preempt traffic signals (extending green lights a little longer if a train is about to pass through) as many other cities do.  Even that, however, would require new equipment that the MBTA cannot afford.  We need a free solution that we can implement with the resources already on hand.

Let’s accept that delays above ground are inevitable but that not all lines will be delayed in the same way at the same time.  Currently, underground stations usually see a fairly steady stream of trains.  Unfortunately, they are not distributed evenly among the four branches. A trio of B trains might pass uselessly by passengers who have been waiting for a D train who then crowd into the next D car when another is right behind it.

Since we accept that delays are inevitable on the inbound trip, we can assume that B and D trains will tend to arrive at Government Center (their last stop) in pairs spaced closer together than when they left Boston College or Riverside.  Currently, those poorly spaced trains will loop at Government Center and start their outbound trips to the same place they originated, incurring more delays all the time.  By Washington Street on the B and C lines, all hope is lost, and trains are frequently sent “express” to the end of the line.

By introducing dynamic looping, delays can be corrected or even eliminated halfway through the trip.

When a train arrives at Government Center, it should begin outbound service on whichever branch needs it most.  If the last B train left 10 minutes ago and a D train left just 3 minutes ago, the next train should make Boston College its destination.  Later, when a pair of delayed B trains arrives together, one can service the Riverside branch.  By routinely reassigning trains in this manner, delays are either balanced across multiple branches, or in some cases eliminated entirely.

Until recently, Type 8 cars couldn’t run on the Riverside branch, so Boston College and Riverside trains were not interchangeable.  However, since at least December, Type 8 cars have been in regular revenue service on both branches.  Now is the perfect time to implement this simple and affordable system.

No passengers would even need to be aware of this new policy.  Both branches begin their service at Government Center, so people waiting on the platform would not have any prior expectation about which train might appear next.

Unfortunately, this is not true for the C and E lines.  Their inbound trains run to North Station and Lechmere respectively.  Even excluding them, dynamic looping can improve service on two lines.  However, with a more dramatic change, we could expand the program to all four branches.

First, all trains could be reassigned when entering Government Center.  Some B and D trains might continue north (as though coming from Heath Street or Cleveland Circle), whereas some C and E trains might discharge their passengers and loop.  This would add complexity that dispatchers would need to unravel, and would confuse (and presumably annoy) many passengers.

Alternatively, all trains might terminate at Government Center.  This would simplify the dispatcher’s job to that of tracking which of the four branches most needs the next train.  Service to the north would come from shuttle trains operating on Government Center’s inner loop — a stretch of track already in place for southbound trains to turn around and return north.

These shuttle trains would be operating entirely on dedicated rights of way (underground or on overpasses), so their schedules should be inherently more reliable than the trains at street level.  This would lessen another major complaint about the Green Line: that service to North Station is too unpredictable (another symptom of the same basic problem).

This is a more dramatic step, of course, whose disadvantages shouldn’t be overlooked.  Someone who works on Beacon Street in Brookline and commutes via the Commuter Rail might be pleased at having more regular and reliable service, but might be displeased at suddenly needing to change trains at Government Center.  This is an area that requires further study from past occasions when the T has changed a line’s terminating station.

Even without the C and E lines, however, a pilot program with just the two branches that already loop at Government Center is worthwhile.  Dynamic looping could make a big difference in performance and passenger satisfaction.  At a time when use of mass transit is rising, but when no funding is available for major capital improvements, this simple solution could go a long way.

It’s time for a change.