I'm a bit confused by your need. Is this trip the only concern?
Is this question only about going from Lauterbrunnen to Grindlewald? How will you be getting to Lauterbrunnen?
The display at Lauterbrunnen will read Interlaken Ost. It will show Zweilütschinen as a stop on the way.
The display at Interlaken Ost will show both Lauterbrunnen and Grindelwald. Half the train goes one place and the other half the other. It is clearly written on the train which is which. It goes at the same times each hour and only follows the same route and always splits at Zweilütschinen.
If you are at Lauterbrunnen and you want to do this journey by train you board the train to Interlaken Ost and get off at Zweilütschinen. Wait for the next train to come up the hill from Interlaken and board the bit that says it is going to Grindelwald.
Pretty easy really. Don't worry about the train numbers, look at the destinations.
I have never heard of a train in Switzerland with a name like 140-Y. Where did you see that?
Many people going from Lauterbrunnen to Grindelwald will take the train to Wengen from Lauterbrunnen, then the gondola up to the ridge, enjoy the view, then take the cable car down into Grindelwald. Slower, but fabulous scenery.
EDIT: It looks like Chris F and I were typing at the same time, with much the same advice.