The "triangle" as such is formed by the three famous Spa towns named after various crowned heads of bygone days:
The three towns that form the "triangle" owe their world renown to over 200 mineral springs that dot the landscape and whose waters have been ascribed healing powers for centuries. During the eighteenth and especially the nineteenth century when railroads made access a lot easier crowned heads, scientists, business people, artists and poets from all over Europe stayed in these spas to cure various ailments or simply to relax. Much of the gorgeous Belle Epoque architecture built when this area was part of Austria Hungary has survived or been reconstructed in the old style. Tourism took a hit during World War I and hadn't recovered when World War II delivered an even more devastating blow. The ethnic German population was mostly forced out after the war and the "iron curtain" severed many ties across the borders. The spas were nationalized and where once crowned heads had relaxed now the sons and daughters of the working class should cure their ailments. After 1990 the border with now reunified Germany became more open and with the entry of the Czech Republic into the EU and then the Schengen Treaty, travel to and from the Spa Triangle has gotten easier than ever before. The now re-privatized Spas try to attract an international clientele and advertise their former glory, but even if you haven't come to cure one of your ailments, simply taking in the gorgeous architecture and the beautiful scenery can be worth a short or long stay.
Naturally all three spa towns are linked to the excellent Czech Railway Network. Franzensbad and Marienbad only have one train station with passenger service while Carlsbad has two.
The mineral water can be used medicinally both by bathing in it and by drinking it. Due to the high content of Sodium Sulfate, it is likely to have laxative effect.
Karlovy Vary is famous for "Becherovka" a liquor named after its inventor. It is a herb based bitter and an acquired taste.