By bus
Buses arrive from Kathmandu and Chitwan in Nepal 36 hr+ and virtually every city in India. Although not as comfortable as the trains, buses are the only choice for some destinations, mainly those in the mountains.
Delhi has a confusing slew of inter-state bus termini ISBT, which all have two names. The Delhi Transport Corporation (http://dtc.nic.in/dt4.htm) is the major operator, but every state also runs its own buses and there are some private operators too.
Kashmere Gate ISBT
aka Maharana Pratap, Metro Kashmere Gate, Line 1/2. This is "the" ISBT and the largest of the lot. Buses to points north, including Nepal.Sarai Kale Khan ISBT
aka Vir Hakikat Rai, next to Hazrat Nizamuddin railway station. Buses to points south.Anand Vihar ISBT
aka Swami Vivekanand, on the east bank of Yamuna. Buses to points east.Bikaner House
bus stop. Buses, including air-conditioned Volvo buses from Jaipur arrive at this place. For travel between Jaipur and Delhi, this bus stop is very clean, less crowded than ISBT, and easy to reach.Majnu ka Tilla
Tibetan colony, a short rickshaw ride from Metro Vidhan Sabha. Buses to Dharamsala.
By train
Trains arrive at one of four main stations: Delhi Junction, also called Old Delhi or Purani Dilli; the second at New Delhi which lies in Central Delhi; Hazrat Nizamuddin a few kilometers to the south; and the upcoming Anand Vihar station to the east very few trains use Delhi Sarai Rohilla or Delhi Cantt stations. Delhi Junction and New Delhi Railway Station are now conveniently connected by Metro Line 2, just minutes apart, while Anand Vihar is served by Line 3. It will take about 40 min-1 hr to travel from the New Delhi Railway Station to the airport by car, depending on traffic.
A ticket office open to all is on the road to Connaught Place with longer hours. It often has waiting times not much longer than at the tourist booking office. You will need to know the number or name of the train you want to take. Easiest of all, though, is to book online through the Indian Railways booking website (http://indianrail.gov.in/). Note, however, that you are required to have both an e-mail address AND a mobile phone number that is registered within India in order to access the booking area of the site.
Once you have purchased a ticket either at the ticket office or online prior to the trip, all you need to do is go to the rail car labeled with your class of service purchased. You can either get on and sit in the first available seat or often times for higher classes of service, they will post a passenger list on the car when it stops. Look for your name and go to the assigned car, cabin and seat. There is never a need to get a boarding pass so if anyone comes out of the crowd to tell you that, don't listen to them; it is a scam. If you're brave, you can simply purchase a general 2nd class ticket and then get on any car where there is availability. The conductor will come by and check your tickets after the train starts moving. If you are in a higher fare class than you are ticketed for, all you have to do is simply pay the difference in fare to the conductor. The only risk here is that the train could be full and you could be stuck in the lowest fare class which can be very crowded with little room to sit.