Delhi to Kathmandu in Nepal is a popular side trip from India (many people also make the journey from Varanasi to Kathmandu). Let's look at the best options for travel from Delhi to Kathmandu, depending on budget.
Delhi to Kathmandu Flights
If you don't mind spending the money, the quickest and easiest way is to fly. Around six different airlines, both low cost and full service, operate on the Delhi to Kathmandu route with departures throughout the day. These include Indian Airlines, Jet Airways, Jet Lite, Spice Jet, Kingfisher Airlines, and Royal Nepal Airways. It's become very competitive, ensuring much lower prices than flying from Varanasi to Kathmandu. Expect to pay around 4,500 rupees ($85) including tax for the cheapest fare. Flying time is around an hour and a half.
Delhi to Kathmandu TrainsAn economical way to travel to from Delhi to Kathmandu is by train to Gorakhpur in Uttar Pradesh, then bus or shared jeep to the border in Sunauli), then another bus or shared jeep to Kathmandu.
There are quite a few trains that run from Delhi to Gorakhpur. However, ideally, you'll want one that arrives very early in the morning. That's because it's around three hours by bus from Gorakhpur to the border, and day buses to Kathmandu stop running by late morning (overnight buses leave in the late afternoon and evening, but they take longer to get there and you'll miss the stunning scenery).
In regards to trains, the Asr Kir Express (#15708) departs Delhi at 3.30 p.m. and arrives in Gorakhpur at 5.50 a.m. Another option with slightly earlier departure and arrival times is the Ndls Njp Sf Exp (#12524). It doesn't run every day though. The fare is 318 rupees in Sleeper Class up to 1,150 rupees in 2AC (more about classes of accommodations on Indian Railways trains). A bus from Sunauli to Kathmandu will cost around 500 rupees upwards.