Long ago ,Around 1956Railways wanted to give good express train between Delhi and mumbai and started it by name Deluxe express 25/26 .later on they started another train from Newdelhi to Howrah also naming it deluxe .but it's number was 81/82 or 103/104 I am not sure
Some time around 1969 Rajhdhani was also started
Now at newdelhi dep of trains was around same time in evening and New delhi had only 7 platforms .it used to give confusion to passenger as one train was going to west and other to East...
more... .so few intelligent people in Railways gave new name as paschim (western Deluxe)for 25/26 and Poorva (Eastern Deluxe)for 81/82& 103/104 exp .
Later on Howrah Deluxe was bifurcated to run 4daysc via Patna and 3 days via Gaya .
Note - As much things I have heard known ,sharing with you