They were building a railway.
1	They	they	PRON	PERS-PL-NOM	Case=Nom|Number=Plur|Person=3|PronType=Prs	3	nsubj	_	_
2	were	be	AUX	PAST-AUX	Mood=Ind|Tense=Past|VerbForm=Fin	3	aux	_	_
3	building	build	VERB	ING	Tense=Pres|VerbForm=Part	0	root	_	_
4	a	a	DET	IND-SG	Definite=Ind|PronType=Art	5	det	_	_
5	railway	railway	NOUN	SG-NOM	Number=Sing	3	obj	_	SpaceAfter=No
6	.	.	PUNCT	Period	_	3	punct	_	_
