Whenever placing either Subway item the game throws a "Placement Obstructed" message, takes the item, and doesn't place anything. I am using several other mods, (45 others) and always throws the message, even with just the 3 Splinter mods online.
A full list of the mods:
5dim Complete Modpack
5Dim's mod - Automated Fuel & Ammo
5Dim's mod - New Automatization
5Dim's mod - New Battlefield
5Dim's mod - New Bot Recaller
5Dim's mod - New Compatibility
5Dim's mod - New Core
5Dim's mod - New Enemies
5Dim's mod - New Energy
5Dim's mod - New Equipment
5Dim's mod - New Infinite Research
5Dim's mod - New Locales
5Dim's mod - New Logistic
5Dim's mod - New Mining
5Dim's mod - New Module
5Dim's mod - New Nuclear
5Dim's mod - New Ores
5Dim's mod - New Resources
5Dim's mod - New Storage
5Dim's mod - New Trains
5Dim's mod - New Transport
5dim-faster start
AAI Containers & Warehouses
AAI Industry
Afraid Of The Dark (enforced personal lights + more)
Alien Biomes
Armoured Biters
Auto Deconstruct
Auto mining
Base Mod
Belt Balancer
Big-Monsters
Bob's Adjustable Inserters mod
Bob's Functions Library mid
Category Integrations for 5Dim
Deadlock's Stacking Beltboxes & Compact Loaders
Dectorio
Disco Science
Disco Science Bridge for 5Dim
Electric Furnaces
Endless resources
Event listener
Factorio Library
Flare Stack
Hall of Fame
Infinite Technology Expanded 2
Inserter Fuel Leech
Jetpack
ModMash Splinter
ModMash Splinter, Assembling Machines
ModMash Splinter, Underground + Subways
ReStack
