The Slovenian Railway Museum occupies former railway facilities north of Ljubljana's centre and preserves locomotives, carriages and railway equipment from Slovenia's rail history. The collection is especially appealing if you like industrial heritage and transport museums with large objects rather than glass-case displays.
It is close enough to the centre for an easy detour, but it feels different from Ljubljana's old-town circuit of bridges, churches and riverside cafes. Check opening times before going, as specialist museums can keep shorter hours than the major national collections.