and its consumption when pulling 20 wagons of 20 tonnes each, goods.

Depends on your train and tracks l suppose, but anyway, this is what wikipedia says:

the AAR claims an energy efficiency of over 400 short ton-miles per gallon of diesel fuel in 2004 (0.588 L/100 km per tonne or 235 J/(km·kg))

So, that would come down to 1 mile per gallon in your case?

Tonnage does not really enter into to it except for the amount of time u have to be in a high throttle position to keep it rolling at a certain speed.A heavy train takes more throttle to keep it going.A new locomotive burns 209 gallons of fuel an hour at full throttle whether it is pulling 2000 tons or 20,000 tons.Here is the chart they give us engineers
Fuel Use
Fuel burn in power (Gallons per hour)
Model HP N8 N7 N6 N5 N4 N3 N2 N1
C44AC 4380 210 171 140 109 79 53 27 12
SD9043AC 4300 202 172 139 100 77 54 28 16
AC Fleet Averge 209 171 140 108 79 53 27 13
Fuel burn in Dynamic Braking
Model HP Idle DB2 DB4 DB6 DB8
C44AC 4380 3 4 6 8 13
SD9043AC 4300 3 7 7 9 9
AC Fleet Averge 3 5 6 8 9

l average 3 gallons per mile using 2 GE CW44AC is pulling 24,000 tons.