loading vs burthen

loading and burthen both are nouns.

Word	Noun	Adjective	Verb	Adverb
loading	Yes	No	No	No
burthen	Yes	No	Yes	No

As nouns, burthen is a hyponym of loading; that is, burthen is a word with a more specific, narrower meaning than loading:

Other hyponyms of loading include dead load, live load, superload, millstone, overburden, overload, overload.

loading (noun)	burthen (noun)
the labor of putting a load of something on or in a vehicle or ship or container etc.	a variant of `burden'
goods carried by a large vehicle
weight to be borne or conveyed
a quantity that can be processed or transported at one time
the ratio of the gross weight of an airplane to some factor determining its lift

loading (verb)	burthen (verb)
	weight down with a load