Here in Europe, when Russian rolling stock arrive at the 1435/1520, the car body is lifted and the 1520 mm bogeys are replaced with 1435 mm bogeys. This method is used for passenger cars; as far as I know, there is no exchange of freight cars from the Russian network to the European one: cargo is transhipped to another train (since we are mostly speaking about containers, this is not as complex as it sounds). Moreover, the Russian loading gauge is larger than the standard European one, so many cars wouldn't be able to run on European lines anyway.
Variable gauge axles are in use at the Spanish/French border, but they are only fitted to light passenger rolling stock. Freight cars swap their 1668 mm bogeys for 1435 mm bogeys.