This paper presents an experimental study on the resistance spot welding of S235JR(Cu) steel sheets used in side walls and roofs of rail vehicle bodies. A timer and current controlled resistance spot welding machine having 120 kVA capacity and a pneumatic application mechanism with a single lever were used to prepare the specimens. Welding duration was chosen to be 5, 10, 15, 20, 25 and 30 periods (1 period = 0.02 s) and also welding currents were increased from 6 kA up to 14 kA in steps of 0.5 kA. The electrode force was kept constant at 6 kN. The obtained welding joints were exposed to tensile peel and tensile shear tests and their nugget sizes were measured by means of an optical microscope. The effect of nugget sizes on tensile peel strength and tensile-shear strength was determined using weld lobe diagrams. Based on this data the optimum welding currents and times were advised.