I wear track suit warm up trousers , I get the fleeced lined , winter I add long underwear ..
a Jersey Knit fabric gives without spandex..
Once I bought a Cycling rain cape, what I wear under it does not have to be waterproof, because the Cape Is..
and as long as it's not raining sideways, my pants stay pretty dry too...
Of course the bike has mudguards , and I have my lights mounted Low , so cape does not drape over them..
(i've seen a picture of a cape with a clear plastic window-patch, so handlebar mounted lights can shine through)
..