"One of the model train's carriages went off the rails and overturned at about 11:00 a.m. [07:00 GMT]. Eight children were on board. Two children were subsequently hospitalized," the source said.
The remaining four children were only slightly injured, suffering "scratches", the source added.
The zoo director, Vladimir Sinitsyn, said: "The Moscow zoo has not seen such an accident for a long time." He also added that the train ride was usually "reliable," and that the cause of the accident could have been something on the rails.