The Train Line Roundabout Cap. In case you don't need a Roundabout with four connections, you can close up a side. Connect with Roundabout by aligning the small pillar in the middle.

You Probably will need unrestricted nudging via the infinite nudge mod to place these (https://ficsit.app/mod/InfiniteNudge)

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Roundabout Connector (https://satisfactory-calculator.com/en/blueprints/index/details/id/7089/name/Roundabout+Connector+%28TL_basic%29)

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This is Part of the Retrofuturistic Railway Collection

(https://satisfactory-calculator.com/en/blueprints/index/details/id/6512/name/Retrofuturistic+Railway+Collection) If not specific to this part please leave any comments there.