The apostrophe would go after the s, so tram drivers', because it is both a possessive and a plural (there are multiple tram drivers), or with no s, because you're using it as an adjective before the noun 'system', so it does not need to be pluralised or have a possessive 's'. I would, however, probably rephrase the title to "a study of a drowsiness detection system for tram drivers", because it sounds very unnatural with so many adjectives stuck together.