It's hard to say specifically without a better understanding of the data, but here's a start.
Are you looking to add the "ID" field from your text file as an attribute in the "routes" feature class attribute table? If so, presumably there is a field in the "template" file that you would like to use? If so, you need to specify those fields in the insert cursor:
# Let's pretend ROUTE_ID is a field already in the template:
cur = arcpy.InsertCursor(routes, ['SHAPE@', 'ROUTE_ID'])
# Then in your loop:
for line in fileinput.input(infile):
# ... geoprocessing steps ...
cur.insertRow([feat, ID])
If you're looking to add a new field to the attribute table, you would just use arcpy.AddField_management before initializing the cursor.
If you're trying to do something totally different than what I assumed, I'd suggest clarifying your question and code so that people can help you out.
Hope this helps...