I don't know what locomotive you have pulling that tank car, but the problem you have sounds like it could be a combination radius/locomotive incompatibility problem. On my first layout years ago, I had 22 inch radius mainlines, 18 on sidings, and a couple of industrial spurs with 15 inch radius. My Athearn Sd45's were intended as strictly mainline power, and only the Sw7 would ever be used to switch the 15 inch radius industry spur, but I decided to do some experimenting which taught me a lot about the dynamics of long locomotives and tight radius. If I tried to back a Sd45 through the 15 inch radius, it made it without problems. If I coupled any freight car to the Sd45 and tried to back it through the 15" radius, the car would tip up on 4 wheels with the other 4 off the tracks as the engine moved through the curve then drop back onto the track when the engine got straightened out. If I coupled a Gp7 to the Sd45 and then coupled a freight car to the Gp7, I could back the entire consist through the tight radius without problems. All of this was done at low speed, and I should mention that each backing move was followed by a forward move, and forward or backward, the results were the same.
In short, I suspect your problem is that you are trying to run a locomotive that is too long for the curve of your chosen turnout, and the engine overhang is pulling that tank car off the rails. Try other cars right behind the locomotive and see if the problem is consistant. If it isn't a problem with other cars, then you need to check the tank car closely against the cars that make it thrugh to see what is different. If it happens with any car coupled behind the particular engine, you have a locomotive/curve radius compatibility problem.
If the problem is only with the tank car, check the following list.
First, check both trucks on the tank car very carefully. Check wheels in gauge? Are the wheels concentric on the axle (when you spin them do they spin in a perfect circle or do they look more like a cam shaft)? Do any wheels wobble on the axle? Check the geometry of both trucks. Are the axles perpendicular to the side frames? Do the trucks side frames run parrallel to the rails, or does the truck kind of "crab walk" sideways down the track.
After carefully checking the trucks completely, go to the car itself and check it out closely. Check it's weight. Is it up to NMRA standards for weight? I've posted the link to the NMRA standards for car weights in all scales.
http://www.nmra.org/standards/rp-20_1.html
Now check the car frame for straightness & plumb. Do you have a solid table with a good flat table top? Put the car on the table, does it sit flat? If the trucks are installed, do all 8 wheels sit flat on the table? Are both trucks inline with each other? In other words, does the cars frame have any warp or twist in it anywhere?
The final test is to check your couplers on the tank car. Do they move freely side to side? Do they have adequate side to side range of movement compared to other freight cars on you layout?
This is a lengthy post, but if you check everything I've mentioned here, I think you will find the answer to your problem.
Sometimes turning a car around will make problems seem to go away. If all you want to do is watch your train run in a circle, that may be a suitable fix. If you want to operate your railroad prototypically, all rolling stock has to operate facing either direction in both pulling and pushing moves on any part of the layout through all track sections. The only exception to this rule is sometimes we make tight radius curves on our model railroads to fit industries that restrict the size of the locomotive that can be used on the local switcher. When we do that, it should be something that we know we did on purpose and we abide by the rule that we only use certain locomotives to switch out those restricted industries. The closest thing the prototype does to that that I know of, is to use empty cars (usually flat cars) as handles to load freight cars on and off barges because barges are not designed to support the weight of a locomotive.