Airlines, especially point to point locos need numbers. The top 50 European cities by population
1 London* United Kingdom 7,556,900 FR Base STN
2 Berlin* Germany 3,425,000 FR Dest SXF
3 Madrid* Spain 3,213,271 FR Base MAD
4 Rome* Italy 2,718,768 FR Base CIA
5 Paris* France 2,188,500 FR Dest BVA
6 Bucharest* Romania 1,931,838
7 Hamburg Germany 1,773,218 FR Dest LBC
8 Warsaw* Poland 1,706,624
9 Budapest* Hungary 1,702,297 FR Dest BUD
10 Vienna* Austria 1,697,937 FR Dest BTS
11 Barcelona Spain 1,615,908 FR Bases GRO,REU,Dest BCN
12 Sofia* Bulgaria 1,402,597
13 Munich Germany 1,356,594
14 Milan Italy 1,299,633 FR Base BGY
15 Prague* Czech Republic 1,223,368 FR Dest PRG
15 Brussels* Belgium 1,080,790 FR Base CRL
16 Birmingham United Kingdom 1,010,200 FR Base BHX
17 Naples Italy 1,004,500
18 Cologne Germany 995,397 FR Base NRN
19 Turin Italy 900,520 FR Dest TRN
20 Marseille France 839,043 FR Base MRS
21 Stockholm* Sweden 810,120 FR Base NYO, Dest VST
22 Valencia Spain 807,200 FR Base VLC (Summer?)
23 Leeds United Kingdom 761,100 FR Dest LBA
24 Kraków Poland 756,666 FR Dest KRK
25 Łódź Poland 750,125 FR Dest LDZ
26 Amsterdam* Netherlands 752,911
27 Athens* Greece 745,514
28 Riga* Latvia 727,578 FR Dest RIX
29 Seville Spain 699,759 FR Dest SVQ
30 Frankfurt Germany 667,598 FR Base HHN
31 Zaragoza Spain 666,129 FR Dest ZAZ
32 Palermo Italy 665,628 FR Dest / Base(soon) PMO
33 Wrocław Poland 633,950 FR Dest WRO
34 Genoa Italy 614,736 FR Dest GOA
35 Copenhagen* Denmark 613,603 FR Dest ( Negotiating)
36 Stuttgart Germany 595,452
37 Dortmund Germany 587,717 FR Base NRN
38 Rotterdam Netherlands 584,058
39 Essen Germany 583,892 FR Base NRN
40 Glasgow United Kingdom 581,900 FR Base PIK
41 Düsseldorf Germany 577,505 FR Base NRN
42 Málaga Spain 566,447 FR Dest
43 Poznań Poland 564,035 FR Dest PZN
44 Vilnius* Lithuania 553,553
45 Bremen Germany 548,477 FR Base BRE
46 Sheffield United Kingdom 530,300
47 Hannover Germany 517,251
48 Leipzig Germany 510,512
49 Dresden Germany 507,513
50 Dublin* Ireland 505,739 Home Sweet Home
Not that many places left to expand to with at least 500,000 population.