Usually if the distance is somewhere around 3 times the diameter of the pile or caisson, there is little or no effect from vibration. As a precaution I would suggest high early strength concrete and a wait time of about 12 hours.
gosai - I concur with Ron's statement that vibration from rail traffic is usually not a problem. As a former bridge contractor, we did several projects adjacent to railroads. Some of it over main lines with 20+ trains per day. No caissons, but plenty of other work that could be considered vulnerable to vibration damage. No problems or other issues.
You have taken care of the most important item... no rail traffic during or immediately after concrete placement. IMHO, proceed with the project and don't worry about it. Concrete is much "tougher" and resistant to abuse than is commonly predicted by theory.