Thailand is a very easy country to get around in. Buses and trains run everywhere, and are usually on-time. Prices are cheap and the level of service is usually quite high. Chicken buses are a rarity, except in the most remote parts of the country, and the traveler on buses and trains will almost certainly enjoy his own seat with enough room to move his legs around. It feels luxurious after coming out of the mountains of Laos or the muddy roads of Cambodia.
The Thai rail system is old, but very functional. Most of the routes run in the north of the country, and a sleeper train is the best way to go to avoid a night in a hotel. The bunks are usually large enough for a 6 foot tall man to spread out and not have to bend his legs. Be advised not to take the top bunk in the non-aircon section. There is virtually no airflow and the train gets steamy. For day-time travel an un-cushioned seat on the train is definitely the cheapest and most enjoyable way to get around.
There are several classes of buses in the country, from the very comfortable double-decker buses with big seats and plenty of leg room that run between major tourist destinations to older, more rickety second class buses that go into the mountains and remoter regions. In general, a traveler will have no difficulty going anywhere in the country by bus and public transport, except the farthest, most remote regions.
Renting a motorbike (or buying one) is another possibility. The Thai police are notorious for collecting big bribes, though, and a foreigner on a motorbike is an easy target. In the rural regions there is more need for private transportation and therefore renting a bike makes sense, but having a bike in a major city is often more of a hassle than a comfort.
Hitchhiking is a possibility for the destitute wanderer, but is often complicated and difficult. Thais are not used to hitchhikers, and will insist on taking you to the local bus station. The police will also often harass the weary drifter, and leave you walking for miles along the subtropical highways in the ungodly heat. Frankly, it’s not a pleasant experience and should only be done in desperation.