Someone's published a guide to London tube stations to show you exactly which carriage you need to be in, in order to be nearest the best platform exit for your onward journey. And I wouldn't be at all surprised if some enterprising Parisian anorak has done the same for the Métro.
We will be travelling via Eurostar to gard du nord and need to continue on to gare du Lyon for TGV connection. There will be 4 of us travelling with probably 2 cases. Do you think we would be better to use RER or taxi?
If you are not loaded down with baggage (2 cases for 4 people or 2 cases each?), the RER D will only take about 7 minutes. If you are not pressed for time bus 65 will also take you to Gare de Lyon with lots to see along the way.
The Seat 61 site should always be examined before taking a train anywhere in Europe. The man in seat 61 is one of the greatest travel resources in Europe, particularly after the passing of the international treasure Ben Haines.