Rake building through wagon building

Dear Team,

I have a problem. I need some helps. I have several coils with weight. I have to load coils on wagons. There are two types of wagons. The capacity of two types of wagons are 64 and 67 respectively. I have to make rakes by joining wagons. There are three types of rakes. In one rake type, 1st type of wagon can be used. It takes 4 wagons to make a rake. In rake type 2, 2nd type of wagon can be used and 5 wagons are required to make a rake. If we club two different types of wagons, then also we can make rake. It takes 3 wagons to make a rake. My objective is to make full rakes and utilize all the coils with minimum cost. 

I have designed a variable v_(i_Rake_Type,i_Wagon_Type,i_Wagon,i_Coil). 

The constraint is 

sum[(i_Wagon_Type,i_Wagon), v_WagonType(i_Rake_Type,'Wagon_1',i_Wagon)] = 4*sum[i_Wagon_Type, v_RakeTypeUsed(i_Rake_Type,'Wagon_1')];

I am finding it sometimes problematic. In the last types of rakes, we need to use two different types of wagons. 

Am I doing in right way. Kindly help.