Table not totalling correctly

There is not enough information here to confirm if this is a bug or not.  It is very common in DAX that the totals don't add up to the individual row items. Your SUMX doesn't appear to iterate over the Train ID, so it is quite possible this is the problem.  Try re-writing your formula like this (to verify if it is the cause).

Total train capacity = SUMX(TableName[Train_ID],SUMX(SUMMARIZE(Vehicle_Movements,Vehicle_Movements[Wagon and number],Vehicle_Movements[Total wagon capacity]),Vehicle_Movements[Total wagon capacity]))

Note I don't know what the table name is that contains the train_ID.  you will have to fix that.  Also I am not saying this is a good formula, but it should tell you if the issue is a bug or a bad formula.

This blog post is a completely different topic, but it explains the "totals don't add up problem".  http://exceleratorbi.com.au/sum-vs-sumx-in-dax/

Sorry if you already know this stuff.