Re: buses or trams

[Nicholas Dow <nikdow@ibm.net>]

Light vs heavy refers to the engineering, not the mode of operation. 
That's why it's a continuum.  
At grade or otherwise is irrelevant.
Roughly speaking, the mass that the tracks are built for is the
criterion.  You can run lighter vehicles on the heavy rail tracks, and
this is how Melbourne's St Kilda & Port Melbourne train lines were
converted to tram operation.

All of Melbourne's heavy rail can take freight trains, even though most
of the suburban lines do not.

The reason you can't mix heavy and light traffic has to do with
collisions and what happens to the "light" vehicle in such an event.  

Greg Gritton wrote:
 
> Finally, what does the "light end" of heavy rail mean.
> Heavy rail cars are often lighter than light rail cars.
> Generally, heavy rail has two characteristics that
> seperate it from "light rail":
> 
>    1. Complete grade seperation* (permits higher speed)
>    2. Long trains                (permits higher capacity)
> 
> Does the type of "metro" you are discussing compromise one of these?
> 
> Greg Gritton
> 
> * Some heavy rail systems might include protected grade crossings,
>   but these are usually rare, and I don't think street
>   running is ever included