This paper analyzes and compares the noise measurement results of rail vehicles obtained by testing with one or two identical trailer units. The test procedure for measurement of the pass-by noise emitted by railway vehicles is generally carried out using the standard method according to ISO 3095, for which it is necessary to perform the measurements with two identical trailer units. In special cases where a series consists of only one unit, i.e. there is no other identical vehicle, testing and signal processing are performed with just one unit. Typically, this is considered an exception. The requirements to provide two identical units can cause certain problems for manufacturers. If the need to modify the prototype arises, it implies changing the two vehicles, which results in increased costs. The paper shows a comparative review of the results and an opinion on the justification of the request to perform tests with two identical trailer units.