The replacement of an existing steel railway bridge in Sweden opened up for fatigue testing connections with a history of about 70 years of in-service loading. One decisive detail in the bridge was the gusset plates welded to the flange edge of I beams. This has low fatigue resistance in the governing standards and was deemed exhausted in theoretical assessments. To estimate the remaining fatigue endurance, 20 similar samples of these were cut out from the bridge and tested. During service, they were exposed to alternating compression and tension stresses. The testing was designed to determine their category for constant amplitude fatigue loading and to evaluate the influence of compressive stresses. It is shown that the compressive stresses have an influence on the fatigue endurance and a correction function has been fitted to the results. Despite many years of loading, they had a fatigue endurances significantly greater than that estimated theoretically. A detail category of 63 was derived, which, together with the correction function, provides a significant increase in fatigue life, which should be possible to use for similar details in other existing steel structures.