Mine Cart with Chest Bug

Basically, I have a long rail runway for my mining dump off point, and the minecart with the chest leads back to the surface. Anyways, every time, the minecart travels different lengths. I found that I the chest was empty it would travel most of the way (most of the time). But if it was full of items, it would only travel half way up, before it needs a power rail to push it up, when it can do it normally when it is empty. When I am personally riding in my minecart up, it always works, but why doesn't it for the minecart chest?