1080Tis (especially Asus ROG ones) are one of the few desktop cards around (apart from Titans) with sufficient horsepower to drive 4K UHD at respectable refresh-rates and frame rates.
BUT: Whether you go for 4k depends a lot on other uses and workflows you have planned for your rig. Depends also on how well your software and OS can scale icons and fonts at such high resolutions.
Fewer than 3% of Steam users (i.e. mostly gamers) run 4K. Most run at 1920 by 1080@60Hz (Full HD/ 1080p) and a lesser number at lower resolutions and there's a slowly growing number of 1440p users.
G-Synch is definitely a cool thing to have - but adds around US$100 to the price of any monitor. Is it worth it? Almost certainly for gamers.
Most people cannot detect differences between 60, 120, 144Hz and higher refresh rates, but elite shoot-em-up gamers swear by, or lust over, those with higher values.
Does Trainz require this? Not always. Wasted on TS12 and below, a high refresh rate monitor at 2k is certainly desirable for T:ANE or TRS2019 (though hardly essential).
Bigger, however is better! So decide what size monitor you want to have grace your desktop/ available space.
Start at 27" (diagonal screen measurement) and work your way up from there; typically with escalating prices for the same features and resolutions.
I'm not a huge fan of curved monitors, but they have their place (mainly for TVs, etc.) but consider achieving the same effect by running more than one monitor side by side. 3 monitors angled to face the user side-by-side are even better.
I like to run T:ANE on one monitor and my monitoring tools and other stuff on the adjacent one. You can stretch the image over two or 3 or even more screens depending on your array using either Asus utilities/ nVidia Settings or Windows 10 Display settings.
T:ANE looks great stretched over two or more similar-sized monitors.
Other stuff to look for- IPS vs TFT and LED/ LCD etc.
Options: 1280 x 720 - HD / 720p - 1920 x 1080 (Full HD/ 1080p) 2560 x 1440 - QHD/WQHD (Quad HD) / 1440p. 3840 x 2160 - UHD (Ultra HD) / 4K 2160p, 8k 7680 by 4320.