I visited this WHS in November 2016. Apart from having railways as WHS, India also has a railway station inscribed on the WH list. That said, what is surprising is not the Terminus's inscription but actually that it is indeed a railway station with some of the most unique railway head offices worldwide.
It is possible to visit the interior of this Victorian Gothic building either simply by accessing the railway station entrance itself or else by booking a Heritage Tour (Monday to Friday, 3-6pm) at the Heritage Museum (ticket issuing time 3-5pm).
This tour not only grants access to the museum at the ground floor but will enable you to view the interior and central dome from the viewing gallery on the 1st floor and the Heritage Lounge on the 2nd floor, apart from a quick peek of the forecourt at ground floor. If you have the time by all means, even though the absolute highlight and OUV of this WHS lies in its magnificent exterior.
Viewing the huge building's exterior can prove quite tricky at times. The rear and lateral parts (station entrance and bus terminus) are quite straightforward to visit and will provide good opportunities to appreciate the different sculptures and architecture. There are 3-4 brass UNESCO plaques proudly displaying Victoria Terminus's inscription as well as a touristy "A UNESCO World Heritage Site" sign on the main gate and facade.
However, the best view of the facade and central dome lies opposite just near the traffic lights in front of the Municipal Corporation Building of Mumbai (known as the BMC). By the way, this building is also worth viewing and is like a continuation of the CST. Best time to visit this spot for photography is in the afternoon as in the morning you'll only be able to see dark silhouettes of the CST. Getting there though involves a walk round the block from one traffic light to the next.
In general, Mumbai was a very pleasant surprise as it has much more charm and well-kept colonial buildings than for example New Delhi and it does not suffer from frequent fog/smog problems like the capital city. Now that I visited both cities, I think I'd choose Mumbai over New Delhi for any short layover I might have in the future. If you want to see this building fully lit, do visit at night time or during the Diwali Festival and you won't regret it.