What is the difference between heavy rail and commuter rail?

Question

Why not just expand heavy rail to the suburbs?

Answer 1

The standards for the road bed are much less for light rail and, consequently, much less exoensive than constructing heavy rail.  Commute equipment is much lighter than what one encounters with a freight train and doesn't need the heavy haul capacity.

By having seperate lines, freight and passenger operations don't get in each other's way, as is often the case with Amtrak running on shared right of way with the freight carriers.

Answer 2

Indeed, commuter rail is typically built to lighter standards since it will not take the heavy loads and daily punishment of constant freight trains (an example of this would be New Mexico's Rail Runner service).  Having said that, some state commuter systems do operate over private or state-owned freight lines.  Another advantage to building commuter systems, if they are to be used expressly and only for such (i.e., meaning no freight operations), is that they are cheaper to build and maintain over traditional freight lines.

Answer 3

Freight, Amtrak and commuter rail, all of which use the same type of railroad.  It falls under FRA jurisdiction and has the familiar knuckle-and-hand couplers you see on freight trains.  It's not wrong to call it heavy rail.

But in a transit context, "heavy rail" usually means subway and subway-like things, such as BART, MARTA, Washington Metro, PATCO, L.A. Red Line etc.  Distinguished by high level platforms, third rail electric power, and total grade separation (no highway crossings) such as long elevated stretches or subways.

Three key differences for commuters

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#1 Cost.  "heavy rail" is massively more expensive to build per mile than commuter rail.  Commuter rail can be cheaply added to existing freight railroads, especially if a city has a nice downtown passenger terminal already.

#2 Construction time.  Building a single subway line is a 10+ year commitment.  But commuter railroads go in fast on existing freight lines.  Los Angeles was able to quickly and cheaply build the sprawling Metrolink system, going from nothing at all to a system that rivals Chicago in just a few years.  In a single week, BART was able to add two demonstration commuter lines from Oakland to Brentwood and Suisun using borrowed L.A. equipment, and then shut them down.

#3 Distance.  50 miles is not unreasonable for a commuter railroad.  They often have bathrooms on board, and food/drink is usually allowed.  50 miles would be miserable on a "heavy rail" line.  BART refuses to believe this, of course :) laying people with grueling 90 minute commutes with no food or bathrooms.