Check Constraint - Compare Sum Of Two Values In One Table Against Values Located In Another Table

I am relatively new to SQL and as a project I have been asked to create the SQL for a simple database to record train details. I want to implement a check constraint which will prevent data from being inserted into a table if the weight of the train is more than the maximum towing weight of the locomotive. FOr instance, I need to add the unladen weight and maximum capacity of each wagon (located in the wagon type table) and compare it against the locomotive maximum pulling weight (the locomotive class table). I have the following SQL but it will not work:

check((select SUM(fwt.unladen_weight+fwt.maximum_payload) from
hauls as h,freight_wagon as fw,freight_wagon_type as fwt,train as t where
h.freight_wagon_serial_number = fw.freight_wagon_serial_number and
fw.freight_wagon_type = fwt.freight_wagon_type and
h.train_number = t.train_number) <
(select lc.maximum_towing_weight from locomotive_class as lc,locomotive as l,train as t where
lc.locomotive_class = l.locomotive_class and l.locomotive_serial_number = t.locomotive_serial_number))

The hauls table is where the constraint has been placed and is the intermediary table between train and freight wagon.

I may not have explained this very well; but in short, i need to compare the sum of two values in one table against a values located in another table...At present I keep getting a message telling me the sub query cannot return more than one row.