hancock4@bbs.cpcn.com wrote:   
   > --lithium ion batteries or "super capacitors" to allow limited off   
   > wire operation* (it still needs tracks).   
   >   
   > *I don't know what a 'super capacitor" is.   
      
   It's better than a regular capacitor.  :)   
      
   A capacitor can store energy sort of like a battery does, but until   
   maybe 15 or 20 years ago, you needed a physically large capacitor to get   
   the equivalent energy storage of a relatively small battery. [1]  Around   
   that time, improved materials and manufacturing started allowing for   
   physically smaller capacitors that have a decent amount of energy   
   storage.   
      
   The other piece of the puzzle is that you need better power electronics   
   to really make use of a supercapacitor.  If you want to get all the   
   energy out of a rechargeable battery, you only have to take it from   
   (say) 12.6 volts when fully charged to (say) 12.0 volts when it's   
   totally dead. [2]  It's pretty easy to design a traction motor that will   
   work well over a 5% change in supply voltage; you can almost just   
   connect the traction motor straight to the battery with no electronics   
   whatsoever.   
      
   If you want to get all the energy out of a supercapacitor, though, you   
   have to take it from (say) 12.6 volts when fully charged, all the way   
   down to 0.0 volts when it's totally dead.  It's harder to design a   
   traction motor that works well over a 100% supply voltage variation.   
   :)  You need a box of electronics in between the supercapacitor and the   
   motor, to accept the varying voltage from the supercapacitor and   
   produce a relatively constant voltage for the motor.  Fortunately,   
   because of electronics developed for variable-frequency drives (in   
   both stationary and traction applications), it's not hard to build the   
   box you need.   
      
   Some super capacitors can also be discharged and charged faster than a   
   battery.  This is interesting if you're trying to move a streetcar off   
   from a stop, or if you want to stop in a hurry but still capture the   
   braking energy for later.  Super capacitors also should last longer   
   than batteries, but I don't know what the real-world experience in   
   traction applications has been.   
      
   > Some trackless trolleys   
      
   When did everybody decide that "trolley bus" was a bad word?  (It's   
   a marketing thing, I know.)   
      
   > Frankly, I didn't really see any substantive differences between them   
   > and traditional light rail.   
      
   Marketing again.  :)   
      
   Matt Roberds   
      
   [1] Super capacitors have been around for longer than 15 or 20 years,   
       but I am talking about when they started to get interesting for   
       traction applications.   
      
   [2] Yes, this is lead-acid, and yes, it's just six cells.  No, you   
       probably wouldn't use lead-acid for a new-build streetcar in 2014.   
       Yes, you would probably use more than six cells of whatever   
       chemistry you did pick.   
      
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