Microshades sleepers are awesome. No need for any lining, just screw them together and away you go. - TheSaltyreefer
Microshades sleepers are excellent and considered "safe". That's what I use in all of our food garden beds here at home. Hardwood, if you can find it untreated, is also excellent but harder to cut, lift, etc. Be aware that a lot of hardwood is CCA treated so look carefully at product details. Some people will suggest recycled railway sleepers. I would advise against that. Apart from being crazy-hard to work with they can be heavily impregnated with all manner of industrial grade chemicals. - Adam_W
The difference between the CCA-treated timber and the Microshades sleeper is that the Microshades one does not contain any arsenic. This makes the Microshades sleeper an ideal timber for use in garden beds.