Loco, In answer to your 511 vs 811 question.....
the 811 is classic period large series O gauge and the 511 is classic period small series standard gauge.
That is some of the "basic" knowledge that the TCA Standard of the World book can give you in a nutshell
as example, just to give a quick overview of prewar flatcars Lionel produced.....
Standard gauge 11, 511, 211 and O gauge 831, 811, 2811, 651, 2651, 3651 and 3811. I think I have them all, if you don't count the Madison hardware "backroom production".
Most series' kept the numbers consistent (all flats end in # 1), but some got thrown off when early O gauge was produced and numbered (800, 801, 802......)
Also the evolution of numbers follows eras and features, such as gauge, automatic couplers and operating mechanisms.
I can't stress enough how much the TCA book will clarify this.