Hi @kirky There are a few differing schools of thought on the use of wall liners. Personally, I will generally only line a bed (or retaining wall) with a drainage or geotextile fabric just to prevent soil from escaping through the gaps. Sleepers can expand & contract with dry and wet weather and also shrink a little as they age so they may be nice & tight when you build the bed but in 12-months time there can be gaps. A lot of folks use raised planters for growing food plants & I know they avoid any form of plastic liners in-case there is the possibility of anything leaching from the plastics into the soil & then being taken up by the plants.
Nice planter, well done! Can I make one suggestion... those uprights in the middle... put a cross brace from side-to-side otherwise the walls will likley bow out. If you go here & scroll through to step 11 you'll see what I mean.