I live in Goa for most of the year and am a frequent train user. Perhaps be aware that the Konkan Railway, largely single line with passing loops, experiences frequent and substantial delays. These have an inevitable knock on effect on journeys further south.
The variety in levels of services offered can be overwhelming,. As a rule of thumb, the higher train numbers (2xxxx services), tend to be prioritized by control and are faster with fewer station stops. Vandhe Bharat and Rajdhani are brands to look out for.
The Seat 61 site gives comprehensive info and is certainly worth a visit. Personally l would avoid 2nd class unreserved unless for journeys lasting under an hour. Also, travel on the Mumbai Suburban system during rush hours is not practicable with bags and not for the faint hearted under any circumstances.
When planning journeys from Mumbai it's worth remembering that southbound services pass through easily accessible PANVEL on the outskirts of the city. Some use this route to bypass Mumbai altogether, so check services from there if other options are full.
Lastly, the 1 2 go site makes it easy to book and pay for Indian train tickets without going through the drama of registering with the IRCTC site
