Factorio Forums

JDLH wrote: Mon Jan 12, 2026 7:56 am Hi, I am running the current version of the Space Age game [version 2.0.72 actually]… and I am baffled by the lane-blocking behaviour of underground exits. I want to block one lane of a basic transport belt, on Gleba, and so I placed an underground belt exit across the belt. Jellynut fruits on both lanes were blocked by the exit. Similarly, spoilage on both lanes was blocked. I expected that the underground belt exit would block only one lane, and would allow the other lane, the one on the side of the exit's discharge belt, to pass.

As requested, here are screenshots of the lane-blocking I see. The jellynut fruits are travelling from left to right. Above, using an underground belt entrance to block the top lane. I expected that the jellynut in the bottom lane would pass the underground exit and continue to the right. I see a jellynut in the bottom lane has made it into the underground entrance's left lane, but it does not pass across the entrance and out the right side. Above, using an underground belt entrance to block the bottom lane. I expected that the jellynut in the top lane would pass the underground exit and continue to the right. Above, using an underground belt exit to block the top lane. I expected that the jellynut in the bottom lane would pass the underground exit and continue to the right. I see a jellynut in the bottom lane has made it into the underground exit's left lane, but it does not pass across the exit and out the right side. Above, using an underground belt exit to block the bottom lane. I expected that the jellynut in the top lane would pass the underground exit and continue to the right. I see a jellynut in the top lane has made it into the underground exit's left lane, but it does not pass across the exit and out the right side.

Do others see the same behaviour?

Should I perhaps report this as a possible bug?

JDLH wrote: Mon Jan 12, 2026 10:32 pm

It will go into the underground belt, not past it—place a matching output belt and they will go through.

Like so:

JDLH wrote: Mon Jan 12, 2026 10:32 pm Do others see the same behaviour?

Yes, you describe how underground belts work and are supposed to work. This is no bug.

Your issue is that you expect the underground belt piece passes items straight to a belt piece on the opposite side. But it doesn't work like this. The underground pieces do sideloading and the items are being moved onto the underground piece. Append a belt pice to the above ground side of the underground exit. Not on the opposite side of the underground exit.