I have used old RR ties for corner post in the past. They worked well for me in my wet clay. But this was years ago so I know nothing about the newer ties they use today.
In fact ever since they made the creosote wood treating plant about 5 miles from my house a super fund cleanup site and had to incinerate 20 acres of land the state has really cracked down on the use of creosote.
For me since it is listed on the restricted use pesticide list I just as soon find something else to use so my kids do not have disposal problems later. Right now the local landfill will not take them and you are not supposed to burn them.
But its your land and kids so do as you feel best.
Quote from EPA
The Agency considered the potential risks to public health posed by Creosote (and the other heavy duty wood preservatives) along with the benefits resulting from their use. As a result of this evaluation, the Agency determined that the use of creosote as a wood preservative chemical met the statutory standard for registration provided that certain risk mitigation measures were implemented. These modifications required that Creosote be classified as restricted use pesticide, workers were required to use certain protection/protective clothing and equipment and Creosote use was restricted to non-residential use sites. These mitigation measures are noted in the Federal Register January 13, 1986 (Vol.51, No. 7).