I am not sure if these have ever been discussed, or discovered but here it goes.

We know of three basic combos for Raiden in MK Trilogy.
HP, HP, LP, LP
HP, HP, LP, F + HP
HK, HK, LK, B + HK
Yet I have found two more. It was a pure accident on how I found them, and the funny thing is they are 3 button presses, but depending on how you do the combo, you will get one less hit one way, and three the other. Below are the two new combos I have found for Raidn in Mortal Kombat Trilogy. This is the normal Raiden, and not the Classic one.

LK, LK, (B + HK)=3 Hit
HK, LK, (B + HK)=2 Hit

I have tested them on all systems, and they work on all systems. Johnny Cage has a hidden combo on the CD Versions only. Most may know this one. It is the same number of button presses as his 5 Hit Combo, only when you do it this way on the CD Versions, you only get 4 Hits for it every time. It is HK, LK, HK, LK, LK.