I found a issue and I confusing about this.
Im playing Minecraft on multiplay server with ImmersiveRailroading-mod. When I put the steam locomotive on the rail, other people cant enter the server any longer until I remove it.
Fix this bug plz....

Can you send me their log files when they can't join?

OK, this is the log when they cant join.
(But I think you cant make out why this error has occurred from this log. If I misunderstand "log" means, please teach me.)
[00:57:48] [Server thread/INFO]: [USERNAME] joined the game
[00:57:49] [Server thread/INFO]: [USERNAME] lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[00:57:49] [Server thread/INFO]: [USERNAME] left the game

No. Actually , I thought it is a weird conflict with another mod, so I put only ImmersiveEngineering, Railroading and TrackAPI on mods folder.

weird

I need their client logs, either fml-client-latest.log or latest.log

Thank you for your suggestion.
Here is "fml-client-latest.log" following;
fml-client-latest.log

PS:
This bug is only caused by "steam locomotive". We can put electromotive or other freight car.

I honestly don't know what's breaking. It looks like a weird conflict with another mod. Is there a crash report from the clients?

I think this is fixed by the latest release, give it a try and let me know if it is still a problem