No. This is not the Mumbai-Goa Tejas Express.
A video of a train all lit up has been going around as a random WhatsApp forward with a caption stating that is that Mumbai-Goa Tejas Express on Konkan Railway all decorated on the occasion of Christmas. The train seen in the video is a steam train with locomotives on either side passing under a bridge.
The fantastic video is neither from Konkan Railway nor from India. To start with steam train operation for regular trains in India stopped in the mid-1970s.
The video is, however, real and is from the famous Dartmouth Steam Railway. Called the Christmas Train of Lights, the vintage carriages of The Devon Train of Lights are decorated with thousands of sparkling lights, creating a magical atmosphere for train passengers as well as spectators watching.
The Dartmouth Steam Railway, formerly known as the Paignton and Dartmouth Steam Railway, is a heritage railway on the former Great Western Railway branch line between Paignton and Kingswear in Devon. The line is owned and operated by Dart Valley Railway Ltd.
The video seen is that of Train of Lights 2021 as it passes through Goodrington, near Paignton. Its original link is HERE.