One point to be made is that Ukrainian broad gauge grain cars are already retrucked at the borders -- at least in Romania -- and operate as unit trains with buffer cars to convert the couplers to European hook and screws at either end. From what I can determine, these are about 40 cars long, so not at a North American level, but they operate. If standard gauge track can extend into Ukraine at a useful distance, you'd avoid the need to re-truck the cars at the border and have at least a temporary pool of cars that can operate in Europe. Certainly Mr Robl is correct that it would take time to build this out substantially, but I can't rule out that counties like Poland have got to be thinking hard about this option.