Given the table of data (which is helpful), Rémi's answer is close to what I would suggest.
SUMX(SUMMARIZE(TableName, [Wagon and Nu], [Total wagon capacity]), [Total wagon capacity])
*changing TableName to the actual name of your table.
The SUMMARIZE creates a filtered table with just the DISTINCT values of [Wagon and Nu] and [Wagon Capacity]. In other words, it gets rid of the duplicate rows (based on the 2 columns listed), then sums the capacity of the remaining unique rows.
In other words:
IB-145 3
IB-358 3
IB-145 3
IB-358 3
is turned into simply
IB-145 3
IB-358 3
And the 2nd column is then summed (which is this small example becomes 6).
If you have any choice over how the data is stored though, I'd recommend splitting your data into two actual tables. In one table, have the ID, [Order Date], Pack, Container, [Wagon & Nu] only. In another table have [Wagon & Nu], Wagon, Number and Capacity split out. In this 2nd table (the lookup table), you only need one row per [Wagon & Nu]. Join the two tables on the key field [Wagon & Nu] and in this situation, a simple SUM on capacity would work.
