OK, I checked a few and steel was the 0th graphic so that would get used; I'm fine with that. But we will need to ensure all long graphics have steel as the 0 graphic rather than wood. I don't think we need an additional graphic for wrought iron.
Hmm - isn't wrought iron significantly darker than steel...?
In any event, steel isn't the 0th graphic in, for example, wagon-long-d1.dat:
obj=vehicle
name=LongD1
#?
speed=56
copyright=James
intro_year=1855
intro_month=1
retire_year=1903
retire_month=5
waytype=track
freight=Bretter
payload=8
length=3
brake_force=0
rolling_resistance=19
#?
weight=4
cost=100000
runningcost=5
sound=-1
min_loading_time=360
max_loading_time=975
Constraint[Next][0]=any
EmptyImage[E]=./images/wagon-long-d1.0.0
EmptyImage[SE]=./images/wagon-long-d1.0.1
EmptyImage[S]=./images/wagon-long-d1.0.2
EmptyImage[SW]=./images/wagon-long-d1.0.3
EmptyImage[W]=./images/wagon-long-d1.0.4
EmptyImage[NW]=./images/wagon-long-d1.0.5
EmptyImage[N]=./images/wagon-long-d1.0.6
EmptyImage[NE]=./images/wagon-long-d1.0.7
freightimagetype[1]=Stahl
FreightImage[1][E]=./images/wagon-long-d1.2.0
FreightImage[1][SE]=./images/wagon-long-d1.2.1
FreightImage[1][S]=./images/wagon-long-d1.2.2
FreightImage[1][SW]=./images/wagon-long-d1.2.3
FreightImage[1][W]=./images/wagon-long-d1.2.4
FreightImage[1][NW]=./images/wagon-long-d1.2.5
FreightImage[1][N]=./images/wagon-long-d1.2.6
FreightImage[1][NE]=./images/wagon-long-d1.2.7
freightimagetype[0]=Bretter
FreightImage[0][E]=./images/wagon-long-d1.1.0
FreightImage[0][SE]=./images/wagon-long-d1.1.1
FreightImage[0][S]=./images/wagon-long-d1.1.2
FreightImage[0][SW]=./images/wagon-long-d1.1.3
FreightImage[0][W]=./images/wagon-long-d1.1.4
FreightImage[0][NW]=./images/wagon-long-d1.1.5
FreightImage[0][N]=./images/wagon-long-d1.1.6
FreightImage[0][NE]=./images/wagon-long-d1.1.7
A similar situation prevails in wagon-long-10t.dat, wagon-ng-long-sar.dat, wagon-long-30t.dat, wagon-long-20t.dat, wagon-long-12t.dat, wagon-ng-bogie-long.dat and wagon-ng-long.dat. The canal barges and road vehicles seem to have steel as the 0th good.
I still suspect that it would be better to have separate images for wrought iron. I had wondered whether it would be possible to produce new graphics quickly using the Gimp, but it seems that it is insufficiently easy to distinguish between the load pixels and the wagon pixels in an automated way to make this work.