Why not just fly directly to Nepal? KA, SQ, TG, and EK all have flights into KTM from their corresponding hubs. The KA service stops, I believe, in DAC as well.

There is no train from China to Nepal. If you want to cross overland from China into Nepal, you'll need to do so by bus (or a private vehicle) at the Zhangmu/Kodari border crossing. To reach this from China, you'll either need to either be a Chinese citizen or arrange for the proper permits and a guided tour. You'll need to arrange transportation on the other side, and it's about five hours to Kathmandu.

Note that this is a one-way border... you can't cross back into China as a foreigner except on a group/tour visa arranged in Nepal, which will cancel any existing Chinese visa you may hold.

Quite simply, it's not worth the hassle... just fly into KTM, and avoid messy Tibet permits and strange visa rules. Even if you also want to visit Tibet, I'd recommend you do so and then simply get a flight to continue to Nepal.