rail 14 provided with two longitudinal grooves 12 and composed of a material which is heat resistant and nonscaling such as for example a copper alloy and which is resistant to temperatures in a range above 450 u00b0 c over long periods of time without undergoing noteworthy changes the rail 14 includes a substantially flat base 16 which represents the heating surface connected with the base are three walls 18 20 22 between which axial grooves 12 are located however it is also possible to provide base 16 with two or more than three upwardly directed walls between which correspondingly less or more grooves extend the outer walls 18 and 22 may for example be bolted to base 16 arranged between walls 18 and 22 and base 16 is one heating element 24 26 each preferably in the form of a rod shaped electrical resistor which extends over the entire length of rail 14 or which may also be divided over the length into several segments so as to enable controlled heating profiles the heating elements 24 26 are provided with plug contacts not shown for their connection to a source of current center wall 20 which is located between outer walls 18 and 22 and extends vertically from base 16 either is integral therewith or it is connected with base 16 in like manner as outer walls.