The railway sector is key to the continuous expansion of industrialized nations, but the sector's working conditions and human performance requirements are qualitatively different from other industries. Human error in railway maintenance is a subject which warrants serious attention so as to achieve and sustain a competitive advantage. This paper investigates the probability of human error during the maintenance process of disc brake assembly unit and wheel set of railway bogie under various error producing conditions in railway maintenance workshop in Luleå, Sweden. The objective is to evaluate human error probability so as to take measures to reduce the likelihood of errors occurring within a system and, thus, to improve the overall levels of safety. For this paper, a case study that explores the causes of maintenance error during disassembly, inspection, maintenance, assembly and installation was derived from brain storming sessions among subject matter experts (SMEs), i.e technicians, supervisors and academic experts. In our case study, the Human Error Assessment and Reduction Technique (HEART) was implemented to evaluate the probability of human error occurring throughout the completion of maintenance task. HEART is based upon the principle that every time a task is performed on the maintenance of a disc brake assembly unit and wheel set, there is a likelihood of failure and the probability of this is affected by one or more error producing condition, for instance, shortage of time, over-riding information, inexperience etc. This paper presents the need for interventions in the human factor elements of maintenance tasks performed on railway bogie. A number of factors directly or indirectly result in a decline in human performance, leading to errors in maintenance tasks. The probability of a technician committing an error during maintenance of the disc brake assembly unit and wheel set is found to be 0.20 and 0.039 respectively. It has been observed that error producing conditions such as time pressure, ability to detect and perceive problems, the existence of over-riding information, the need to make absolute decisions, and a mismatch between the operator and the designer's model are major contributors to human error.