A few corrections Matt D. Heavy Rail does not have to be locomotives pulling commuter coaches. Subway is considered Heavy-Rail. Multiple Units are also considered heavy rail.
Also, they do not have to be diesel locomotives. It can be either.
Here is a Locomotive-hauled Heavy-Rail in Austria. As you can see, the locomotive is electric:
These perticular locomotives are built by Siemens of Germany. They're one of the most powerful and one of the fastest locomotives in the world. They're capable of reaching 150mph, you'll never see a diesel locomotive reach taht speed.
Here is a Multiple Unit, another example of Heavy-Rail:
As you can see, they are also electric. In Europe, around 90% of rail is electric.
By the way, Light-Rail uses standard gauge just like Heavy-Rail, no difference.
Here is Denver's Light-Rail:
This is the 3 mile part of the line that runs through the downtown streets:
Regarding costs, Heavy-Rail costs more than twice as much to operate than Light-Rail, you simply can't compre.
Light-Rail and Heavy-Rail are for different purposes, you really can't compare them.
Light-Rail is an alternative to buses.
Light-Rail is basically a bus on rails. They're designed to meet the maximum width limit so they can be compatible on streets. The point of Light-Rail is to allow rail transit to share the road with other cars.
Also, regarding diesel power vs. electric power. Electric is by far superior in EVERY way.
Electric accelerates a lot quicker, is MUCH faster, MUCH more powerful, and of course MUCH more quieter.
Most diesel trains nowadays can max only 80-90mph max. However, electric trains can well exceed 150mph easily. There are VERY few diesel trains which can exceed 100mph.
The reason for this is electric trains recieve limitless power which comes from overhead wires, while diesel trains rely soley on the power generated by a diesel engine.
Let me explain to you how diesel trains work. Diesel trains are really "diesel-electric".
This means they have an Electric AC or DC motor just like a regular electric train. But instead of recieving power from overhead electric lines, they recieve power from the diesel engine.
So they are basically electric trains with built in powerplants
HOWEVER, electric costs a lot more to operate, and a lot more to build. Building an electrified line vs. a line that is not electrifed costs more than twice as much. The electric wires themselves need to be checked and maintained often.
Electric trains cost more than twice as much to build, run, and maintain than diesel trains. This is why most of the US uses diesel power.
The European rail industry has been thriving since the beginning of rail, therefore the rail industry in Europe is rich. They can afford electrifying the lines.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran