For your Nong Khai–Bangkok travel you have a choice of a bus or a train. Trains take a couple of hours longer to arrive to the destination, but the night sleepers give you a great chance to sleep the whole journey through. For solo women travellers there is an excellent option of ladies only sleepers. Buses are faster, and the choice is wider as they depart throughout the day. If Nong Khai is your first port of call after Laos and you are planning to take the international bus from Vientiane to Nong Khai bus station, it is easier to proceed with a bus, though you can as well get off the international bus after going though the Thai immigration and walk to the train station instead.
Tip: Buses from Nong Khai arive to the northern Bus Terminal in Bangkok, Morchit. If you are going to stay in Bangkok, you will have to make rather a long journey to the centre or the most popular areas with a wide choice of accommodation. Trains bring you to the centrally located Hua Lamphong train station which is handier. If you are going to head to Suvarnabhumi or Don Mueang airports, it does not make much difference whether to arrive to Mochit or Hua Lamphong. There are shuttle buses to both airports from Morchit. From the train station you can take the Metro (Phetchaburi station) to connect to Makkasan station of the skyrail (operating hours 6am–12.25am daily). There are regular direct commuter trains to Don Mueang airport, which take about ¾ hr to reach the airport. You can buy tickets to Suvarnabhumi from Hua Lamphong with 12asia, too.
From Nong Khai to Bangkok by train
Nong Khai train station is the terminus of the northern line of the state railway of Thailand. The train station is located an easy 15-minute walk or a short tuk-tuk ride from the Friendship bridge. There are four trains heading to the capital during the day, each taking from 11 to 13 hours to cover the whole distance between the cities.
The morning train #76 departs from Nong Khai at 7am and arrives to Hua Lamphong train station in Bangkok at 6.10pm. It is a second class AC seats only at THB748 and it actually steals you the whole day. But it is a decent if you are going to get off on the way.
There are two other trains with seats only: a 6.15pm train #78 arriving to Bangkok at 5am (3rd class AC seats at THB748) and a 6.30pm train #134, the slowest (nearly 13 hours to arrive to Bangkok) and the cheapest one (THB413) from the bunch with third class fan seats only. If you can, try to avoid both of them.
The most convenient option is the #26 sleeper at 7.10pm. It reaches Hua Lamphong at 6.50am and offers first (THB1,807) and second (THB1,248) class coaches, as well as second-class ladies only (TH1,248) coaches. The night train is a great choice as you can sleep with more comfort compared to a bus, though the latter are also quite comfortable.
From Nong Khai to Bangkok by bus
Buses to Bangkok by various private bus companies depart during the day. You may try to buy a ticket on the spot at the bus station in Nong Khai where the international bus from Vientiane arrives. Buses differ by price and the level of comfort they offer. For longer trips we recommend to opt for VIP express buses as the price difference is not great, but the level of comfort is.
Budsarakham Tour operates a VIP bus which departs from Nong Khai at 8.05pm and arrives to the Mochit Bus Terminal in Bangkok early in the morning at 4.35am. The service is fast and reasonably prices (THB562). Snacks are provided on board and the bus makes a couple of pit stops at large gas stations with a variety of forecourt operators, from hawkers to chain restaurants and 7/Eleven convenience stores.
Tip: Nong Khai has good bus connections with the other towns and cities of Isan and other provinces of Thailand. If you are not fond of 8+ hours bus trips, split you journey making a detour to Loei, Khon Kaen or Kanchanaburi.