itinerary - sing.plur
Is it correct to say trains itinerary to mean the routes of the trains, or the word is just route ?

And are these expression always singular and why?

Three weeks is a long time
100 pounds is a lot of money
100 litres of oil was spilled

Question

itinerary - sing.plur
Is it correct to say trains itinerary to mean  the routes of the trains, or the word is  just route ?

And  are these expression always singular and why?

Three weeks is a long time
100 pounds is a lot of money
100 litres of oil was spilled

Answer

An itinerary is a travel plan.  You can say "train itinerary" or "train itineraries" (compound noun rules here), but I can't say that it's the same as "route". "Itinerary" can include several trains which make up a full journey.

Good question about the plurals being treated as singular. We do this to mean the plural functions as a single unit, ie. "in total".  If you use the plural are/were, then I'd think that each week/pound/litre was separate.

Answer

When you're dealing with numbers and measurements even distance, it should be singular.