At this time - October 2011 - there is no train service from New Jalpaiguri to Darjeeling.  This is due to recent landslides and an earthquake, so Darjeeling is effectively isolated.  Optimistic as they are, the Darjeeling Himalayan Railway [DHR] are not currently offering any target date for the completion of repairs. If you are intending to visit Darjeeling, you should first check the DHR website for their latest news and views.  Certain trains do run either side of the landslips, so those might be of some benefit.  

The only alternative to the train service is the road up to Darjeeling and everything has to go by that!  What does this mean?  Whatever the distances are between Bagdogra Airport/New Jalpaiguri and Darjeeling it will take 5 hours approximately, discounting any stops because state highway NH55 similarly suffers from landslides. Once this road starts to climb, its surface becomes tortuous with continuous bumping and crashing and bottom gear crawls.  Public buses may be marginally more comfortable with their large wheels and high chassis but their bench seating may still make it pretty sickening and they frequently get delayed at hold-ups between large trucks etc of which there are many on these narrow roads. Of course the many 4x4 taxis will make the journey somewhat quicker.  Drivers see a reasonable load as being 12 people plus toddlers and luggage .  Presumably if you wish to travel with fewer passengers you can negotiate this with the driver.  Thus, if rough roading in 4x4s is your idea of fun, this could well be your event!  For added excitement drivers will allow passengers to stand on the back bumper or sit up on the roof rack with the luggage.  In the hills there is little - especially at the landslides - to stop you going 'off-road' which means: straight down - a long way!  A suggestion before you embark on such excitement:  Go into the 4x4 taxi ranks and have a close look at the vehicles.  They will gleam with attention to the body work but do also look at the tires which might be bald and patched.  How safe will you be in a skid and is there a spare tire? Is all the vehicle lighting working?  Generally speaking, if what you can see looks dodgy, what about the brakes and steering?  There are regulations but how strict they are is not obvious.  Nevertheless if you feel it is all part of your fun - enjoy yourself!  
Is Darjeeling worth all this effort?  As a hill town it is not the architectural novelty that Shimla is. The Race Track is now used by the Army but do check the latest entries in 'Things to do in Darjeeling'.