Coolwind,
Hi Greg. I have used RR ties for years at my cabin and at my parents cabin. I have a stack of 50 or 60 of them at my cabin that I use for all sorts of projects. I constructed a stairway using 32 - 34 old RR ties built right into the river bank. When it was first completed, if it rained really heavy, you could see a very light sheen on the water surface (like you would from a couple drops of motor oil). But, after 20 years, this has all leached away and rubbed off.
You probably have some very old, used RR ties at your disposal or can get them from a garden center for landscaping. These ties, if they are truly RR ties, were probably used for many years and most of the creosote on and near the very surface has also been washed out or rubbed off by time and the elements. You won't see much effect around your pond from them. Although, I certainly wouldn't put them actually in a pond or allow the water that runs over them to enter the pond. I doubt that it would affect the fish in any way, but there may still be enough creosote and other contaminants to make the surface of your pond have that oil sheen which may be displeasing for aesthetics. Then again, it might look kinda pretty. Mosquitos and their larvae wouldn't like it, that's for certain.
Since your intention is to place them under the liner, I see no problem what-so-ever with using them. I wouldn't use brand new RR ties, though. Those would leach out the creosote and other compounds quite a bit and they will smell really awful when it is hot outside. They can also burn your skin quite a bit. Freshly creosoted ties are quite nasty to work with and you need gloves and a long-sleeved shirt. Speaking of burning, they also burn... as in "rip-roaring, flaming fire burn" if they ever ignite. If they do catch fire, they are extremely difficult to put out! But that's the brand new ties that the RR uses, not landscaping ties. The very old and weathered RR ties for landscaping are more like regular wood in regards to fire.
Gordy