I don't know of a waterproof ink that you can use with a fountain pen like the Rotring Art Pen. But that isn't a great problem you can create washes from the pen lines with a Pentel wash brush or a wet watercolour brush.
Although a pen is best for sketching buildings I much prefer to sketch directly with a brush for street scenes. I drew the sketch of Napier with a sepia Pentel Colour Brush. The brush marks can treated with a wet watercolour brush to create washes.